2005-04-17 00:20:36 +02:00
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Documentation for /proc/sys/fs/* kernel version 2.2.10
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(c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
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2009-04-03 01:57:20 +02:00
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(c) 2009, Shen Feng<shen@cn.fujitsu.com>
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2005-04-17 00:20:36 +02:00
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For general info and legal blurb, please look in README.
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==============================================================
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This file contains documentation for the sysctl files in
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/proc/sys/fs/ and is valid for Linux kernel version 2.2.
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The files in this directory can be used to tune and monitor
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miscellaneous and general things in the operation of the Linux
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kernel. Since some of the files _can_ be used to screw up your
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system, it is advisable to read both documentation and source
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before actually making adjustments.
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2009-04-03 01:57:20 +02:00
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1. /proc/sys/fs
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----------------------------------------------------------
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2005-04-17 00:20:36 +02:00
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Currently, these files are in /proc/sys/fs:
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2009-04-03 01:57:20 +02:00
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- aio-max-nr
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- aio-nr
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2005-04-17 00:20:36 +02:00
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- dentry-state
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- dquot-max
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- dquot-nr
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- file-max
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- file-nr
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- inode-max
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- inode-nr
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- inode-state
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2008-02-06 10:37:16 +01:00
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- nr_open
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2005-04-17 00:20:36 +02:00
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- overflowuid
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- overflowgid
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2016-01-18 16:36:09 +01:00
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- pipe-user-pages-hard
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- pipe-user-pages-soft
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fs: add link restrictions
This adds symlink and hardlink restrictions to the Linux VFS.
Symlinks:
A long-standing class of security issues is the symlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like /tmp. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given symlink (i.e. a
root process follows a symlink belonging to another user). For a likely
incomplete list of hundreds of examples across the years, please see:
http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
The solution is to permit symlinks to only be followed when outside
a sticky world-writable directory, or when the uid of the symlink and
follower match, or when the directory owner matches the symlink's owner.
Some pointers to the history of earlier discussion that I could find:
1996 Aug, Zygo Blaxell
http://marc.info/?l=bugtraq&m=87602167419830&w=2
1996 Oct, Andrew Tridgell
http://lkml.indiana.edu/hypermail/linux/kernel/9610.2/0086.html
1997 Dec, Albert D Cahalan
http://lkml.org/lkml/1997/12/16/4
2005 Feb, Lorenzo Hernández García-Hierro
http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html
2010 May, Kees Cook
https://lkml.org/lkml/2010/5/30/144
Past objections and rebuttals could be summarized as:
- Violates POSIX.
- POSIX didn't consider this situation and it's not useful to follow
a broken specification at the cost of security.
- Might break unknown applications that use this feature.
- Applications that break because of the change are easy to spot and
fix. Applications that are vulnerable to symlink ToCToU by not having
the change aren't. Additionally, no applications have yet been found
that rely on this behavior.
- Applications should just use mkstemp() or O_CREATE|O_EXCL.
- True, but applications are not perfect, and new software is written
all the time that makes these mistakes; blocking this flaw at the
kernel is a single solution to the entire class of vulnerability.
- This should live in the core VFS.
- This should live in an LSM. (https://lkml.org/lkml/2010/5/31/135)
- This should live in an LSM.
- This should live in the core VFS. (https://lkml.org/lkml/2010/8/2/188)
Hardlinks:
On systems that have user-writable directories on the same partition
as system files, a long-standing class of security issues is the
hardlink-based time-of-check-time-of-use race, most commonly seen in
world-writable directories like /tmp. The common method of exploitation
of this flaw is to cross privilege boundaries when following a given
hardlink (i.e. a root process follows a hardlink created by another
user). Additionally, an issue exists where users can "pin" a potentially
vulnerable setuid/setgid file so that an administrator will not actually
upgrade a system fully.
The solution is to permit hardlinks to only be created when the user is
already the existing file's owner, or if they already have read/write
access to the existing file.
Many Linux users are surprised when they learn they can link to files
they have no access to, so this change appears to follow the doctrine
of "least surprise". Additionally, this change does not violate POSIX,
which states "the implementation may require that the calling process
has permission to access the existing file"[1].
This change is known to break some implementations of the "at" daemon,
though the version used by Fedora and Ubuntu has been fixed[2] for
a while. Otherwise, the change has been undisruptive while in use in
Ubuntu for the last 1.5 years.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/linkat.html
[2] http://anonscm.debian.org/gitweb/?p=collab-maint/at.git;a=commitdiff;h=f4114656c3a6c6f6070e315ffdf940a49eda3279
This patch is based on the patches in Openwall and grsecurity, along with
suggestions from Al Viro. I have added a sysctl to enable the protected
behavior, and documentation.
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2012-07-26 02:29:07 +02:00
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- protected_hardlinks
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- protected_symlinks
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2006-08-27 10:23:28 +02:00
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- suid_dumpable
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2005-04-17 00:20:36 +02:00
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- super-max
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- super-nr
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2009-04-03 01:57:20 +02:00
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==============================================================
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aio-nr & aio-max-nr:
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aio-nr is the running total of the number of events specified on the
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io_setup system call for all currently active aio contexts. If aio-nr
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reaches aio-max-nr then io_setup will fail with EAGAIN. Note that
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raising aio-max-nr does not result in the pre-allocation or re-sizing
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of any kernel data structures.
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2005-04-17 00:20:36 +02:00
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==============================================================
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dentry-state:
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From linux/fs/dentry.c:
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--------------------------------------------------------------
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struct {
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int nr_dentry;
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int nr_unused;
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int age_limit; /* age in seconds */
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int want_pages; /* pages requested by system */
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int dummy[2];
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} dentry_stat = {0, 0, 45, 0,};
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--------------------------------------------------------------
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Dentries are dynamically allocated and deallocated, and
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nr_dentry seems to be 0 all the time. Hence it's safe to
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assume that only nr_unused, age_limit and want_pages are
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used. Nr_unused seems to be exactly what its name says.
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Age_limit is the age in seconds after which dcache entries
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can be reclaimed when memory is short and want_pages is
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nonzero when shrink_dcache_pages() has been called and the
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dcache isn't pruned yet.
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==============================================================
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dquot-max & dquot-nr:
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The file dquot-max shows the maximum number of cached disk
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quota entries.
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The file dquot-nr shows the number of allocated disk quota
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entries and the number of free disk quota entries.
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If the number of free cached disk quotas is very low and
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you have some awesome number of simultaneous system users,
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you might want to raise the limit.
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==============================================================
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file-max & file-nr:
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The value in file-max denotes the maximum number of file-
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handles that the Linux kernel will allocate. When you get lots
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of error messages about running out of file handles, you might
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want to increase this limit.
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2011-03-16 00:12:05 +01:00
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Historically,the kernel was able to allocate file handles
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dynamically, but not to free them again. The three values in
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file-nr denote the number of allocated file handles, the number
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of allocated but unused file handles, and the maximum number of
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file handles. Linux 2.6 always reports 0 as the number of free
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file handles -- this is not an error, it just means that the
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number of allocated file handles exactly matches the number of
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used file handles.
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2009-09-24 00:56:13 +02:00
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Attempts to allocate more file descriptors than file-max are
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reported with printk, look for "VFS: file-max limit <number>
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reached".
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2005-04-17 00:20:36 +02:00
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==============================================================
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2008-02-06 10:37:16 +01:00
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nr_open:
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This denotes the maximum number of file-handles a process can
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allocate. Default value is 1024*1024 (1048576) which should be
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enough for most machines. Actual limit depends on RLIMIT_NOFILE
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resource limit.
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==============================================================
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2005-04-17 00:20:36 +02:00
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inode-max, inode-nr & inode-state:
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As with file handles, the kernel allocates the inode structures
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dynamically, but can't free them yet.
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The value in inode-max denotes the maximum number of inode
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handlers. This value should be 3-4 times larger than the value
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in file-max, since stdin, stdout and network sockets also
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need an inode struct to handle them. When you regularly run
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out of inodes, you need to increase this value.
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The file inode-nr contains the first two items from
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inode-state, so we'll skip to that file...
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Inode-state contains three actual numbers and four dummies.
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The actual numbers are, in order of appearance, nr_inodes,
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nr_free_inodes and preshrink.
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Nr_inodes stands for the number of inodes the system has
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allocated, this can be slightly more than inode-max because
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Linux allocates them one pageful at a time.
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Nr_free_inodes represents the number of free inodes (?) and
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preshrink is nonzero when the nr_inodes > inode-max and the
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system needs to prune the inode list instead of allocating
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more.
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==============================================================
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overflowgid & overflowuid:
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Some filesystems only support 16-bit UIDs and GIDs, although in Linux
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UIDs and GIDs are 32 bits. When one of these filesystems is mounted
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with writes enabled, any UID or GID that would exceed 65535 is translated
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to a fixed value before being written to disk.
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These sysctls allow you to change the value of the fixed UID and GID.
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The default is 65534.
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==============================================================
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2016-01-18 16:36:09 +01:00
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pipe-user-pages-hard:
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Maximum total number of pages a non-privileged user may allocate for pipes.
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Once this limit is reached, no new pipes may be allocated until usage goes
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below the limit again. When set to 0, no limit is applied, which is the default
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setting.
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==============================================================
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pipe-user-pages-soft:
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Maximum total number of pages a non-privileged user may allocate for pipes
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before the pipe size gets limited to a single page. Once this limit is reached,
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new pipes will be limited to a single page in size for this user in order to
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limit total memory usage, and trying to increase them using fcntl() will be
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denied until usage goes below the limit again. The default value allows to
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allocate up to 1024 pipes at their default size. When set to 0, no limit is
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applied.
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==============================================================
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2005-04-17 00:20:36 +02:00
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fs: add link restrictions
This adds symlink and hardlink restrictions to the Linux VFS.
Symlinks:
A long-standing class of security issues is the symlink-based
time-of-check-time-of-use race, most commonly seen in world-writable
directories like /tmp. The common method of exploitation of this flaw
is to cross privilege boundaries when following a given symlink (i.e. a
root process follows a symlink belonging to another user). For a likely
incomplete list of hundreds of examples across the years, please see:
http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
The solution is to permit symlinks to only be followed when outside
a sticky world-writable directory, or when the uid of the symlink and
follower match, or when the directory owner matches the symlink's owner.
Some pointers to the history of earlier discussion that I could find:
1996 Aug, Zygo Blaxell
http://marc.info/?l=bugtraq&m=87602167419830&w=2
1996 Oct, Andrew Tridgell
http://lkml.indiana.edu/hypermail/linux/kernel/9610.2/0086.html
1997 Dec, Albert D Cahalan
http://lkml.org/lkml/1997/12/16/4
2005 Feb, Lorenzo Hernández García-Hierro
http://lkml.indiana.edu/hypermail/linux/kernel/0502.0/1896.html
2010 May, Kees Cook
https://lkml.org/lkml/2010/5/30/144
Past objections and rebuttals could be summarized as:
- Violates POSIX.
- POSIX didn't consider this situation and it's not useful to follow
a broken specification at the cost of security.
- Might break unknown applications that use this feature.
- Applications that break because of the change are easy to spot and
fix. Applications that are vulnerable to symlink ToCToU by not having
the change aren't. Additionally, no applications have yet been found
that rely on this behavior.
- Applications should just use mkstemp() or O_CREATE|O_EXCL.
- True, but applications are not perfect, and new software is written
all the time that makes these mistakes; blocking this flaw at the
kernel is a single solution to the entire class of vulnerability.
- This should live in the core VFS.
- This should live in an LSM. (https://lkml.org/lkml/2010/5/31/135)
- This should live in an LSM.
- This should live in the core VFS. (https://lkml.org/lkml/2010/8/2/188)
Hardlinks:
On systems that have user-writable directories on the same partition
as system files, a long-standing class of security issues is the
hardlink-based time-of-check-time-of-use race, most commonly seen in
world-writable directories like /tmp. The common method of exploitation
of this flaw is to cross privilege boundaries when following a given
hardlink (i.e. a root process follows a hardlink created by another
user). Additionally, an issue exists where users can "pin" a potentially
vulnerable setuid/setgid file so that an administrator will not actually
upgrade a system fully.
The solution is to permit hardlinks to only be created when the user is
already the existing file's owner, or if they already have read/write
access to the existing file.
Many Linux users are surprised when they learn they can link to files
they have no access to, so this change appears to follow the doctrine
of "least surprise". Additionally, this change does not violate POSIX,
which states "the implementation may require that the calling process
has permission to access the existing file"[1].
This change is known to break some implementations of the "at" daemon,
though the version used by Fedora and Ubuntu has been fixed[2] for
a while. Otherwise, the change has been undisruptive while in use in
Ubuntu for the last 1.5 years.
[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/linkat.html
[2] http://anonscm.debian.org/gitweb/?p=collab-maint/at.git;a=commitdiff;h=f4114656c3a6c6f6070e315ffdf940a49eda3279
This patch is based on the patches in Openwall and grsecurity, along with
suggestions from Al Viro. I have added a sysctl to enable the protected
behavior, and documentation.
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
2012-07-26 02:29:07 +02:00
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|
|
protected_hardlinks:
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|
|
|
|
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|
A long-standing class of security issues is the hardlink-based
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|
|
time-of-check-time-of-use race, most commonly seen in world-writable
|
|
|
|
directories like /tmp. The common method of exploitation of this flaw
|
|
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is to cross privilege boundaries when following a given hardlink (i.e. a
|
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root process follows a hardlink created by another user). Additionally,
|
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on systems without separated partitions, this stops unauthorized users
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from "pinning" vulnerable setuid/setgid files against being upgraded by
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the administrator, or linking to special files.
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When set to "0", hardlink creation behavior is unrestricted.
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When set to "1" hardlinks cannot be created by users if they do not
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already own the source file, or do not have read/write access to it.
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This protection is based on the restrictions in Openwall and grsecurity.
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|
==============================================================
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protected_symlinks:
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A long-standing class of security issues is the symlink-based
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time-of-check-time-of-use race, most commonly seen in world-writable
|
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|
directories like /tmp. The common method of exploitation of this flaw
|
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|
is to cross privilege boundaries when following a given symlink (i.e. a
|
|
|
|
root process follows a symlink belonging to another user). For a likely
|
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|
incomplete list of hundreds of examples across the years, please see:
|
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|
|
http://cve.mitre.org/cgi-bin/cvekey.cgi?keyword=/tmp
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When set to "0", symlink following behavior is unrestricted.
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When set to "1" symlinks are permitted to be followed only when outside
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a sticky world-writable directory, or when the uid of the symlink and
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follower match, or when the directory owner matches the symlink's owner.
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|
This protection is based on the restrictions in Openwall and grsecurity.
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|
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|
==============================================================
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|
2006-08-27 10:23:28 +02:00
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suid_dumpable:
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This value can be used to query and set the core dump mode for setuid
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or otherwise protected/tainted binaries. The modes are
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0 - (default) - traditional behaviour. Any process which has changed
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2012-07-30 23:39:15 +02:00
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privilege levels or is execute only will not be dumped.
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2006-08-27 10:23:28 +02:00
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1 - (debug) - all processes dump core when possible. The core dump is
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owned by the current user and no security is applied. This is
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intended for system debugging situations only. Ptrace is unchecked.
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2012-07-30 23:39:15 +02:00
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This is insecure as it allows regular users to examine the memory
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contents of privileged processes.
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2006-08-27 10:23:28 +02:00
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2 - (suidsafe) - any binary which normally would not be dumped is dumped
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2012-07-30 23:39:15 +02:00
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anyway, but only if the "core_pattern" kernel sysctl is set to
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either a pipe handler or a fully qualified path. (For more details
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on this limitation, see CVE-2006-2451.) This mode is appropriate
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when administrators are attempting to debug problems in a normal
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|
environment, and either have a core dump pipe handler that knows
|
|
|
|
to treat privileged core dumps with care, or specific directory
|
|
|
|
defined for catching core dumps. If a core dump happens without
|
|
|
|
a pipe handler or fully qualifid path, a message will be emitted
|
|
|
|
to syslog warning about the lack of a correct setting.
|
2006-08-27 10:23:28 +02:00
|
|
|
|
|
|
|
==============================================================
|
|
|
|
|
2005-04-17 00:20:36 +02:00
|
|
|
super-max & super-nr:
|
|
|
|
|
|
|
|
These numbers control the maximum number of superblocks, and
|
|
|
|
thus the maximum number of mounted filesystems the kernel
|
|
|
|
can have. You only need to increase super-max if you need to
|
|
|
|
mount more filesystems than the current value in super-max
|
|
|
|
allows you to.
|
|
|
|
|
|
|
|
==============================================================
|
|
|
|
|
|
|
|
aio-nr & aio-max-nr:
|
|
|
|
|
|
|
|
aio-nr shows the current system-wide number of asynchronous io
|
|
|
|
requests. aio-max-nr allows you to change the maximum value
|
|
|
|
aio-nr can grow to.
|
|
|
|
|
|
|
|
==============================================================
|
2009-04-03 01:57:20 +02:00
|
|
|
|
2016-09-28 07:27:17 +02:00
|
|
|
mount-max:
|
|
|
|
|
|
|
|
This denotes the maximum number of mounts that may exist
|
|
|
|
in a mount namespace.
|
|
|
|
|
|
|
|
==============================================================
|
|
|
|
|
2009-04-03 01:57:20 +02:00
|
|
|
|
|
|
|
2. /proc/sys/fs/binfmt_misc
|
|
|
|
----------------------------------------------------------
|
|
|
|
|
|
|
|
Documentation for the files in /proc/sys/fs/binfmt_misc is
|
2017-10-03 23:16:37 +02:00
|
|
|
in Documentation/admin-guide/binfmt-misc.rst.
|
2009-04-03 01:57:20 +02:00
|
|
|
|
|
|
|
|
|
|
|
3. /proc/sys/fs/mqueue - POSIX message queues filesystem
|
|
|
|
----------------------------------------------------------
|
|
|
|
|
|
|
|
The "mqueue" filesystem provides the necessary kernel features to enable the
|
|
|
|
creation of a user space library that implements the POSIX message queues
|
|
|
|
API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System
|
|
|
|
Interfaces specification.)
|
|
|
|
|
|
|
|
The "mqueue" filesystem contains values for determining/setting the amount of
|
|
|
|
resources used by the file system.
|
|
|
|
|
|
|
|
/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the
|
|
|
|
maximum number of message queues allowed on the system.
|
|
|
|
|
|
|
|
/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the
|
|
|
|
maximum number of messages in a queue value. In fact it is the limiting value
|
|
|
|
for another (user) limit which is set in mq_open invocation. This attribute of
|
|
|
|
a queue must be less or equal then msg_max.
|
|
|
|
|
|
|
|
/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the
|
|
|
|
maximum message size value (it is every message queue's attribute set during
|
|
|
|
its creation).
|
|
|
|
|
2012-06-01 01:26:33 +02:00
|
|
|
/proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the
|
|
|
|
default number of messages in a queue value if attr parameter of mq_open(2) is
|
|
|
|
NULL. If it exceed msg_max, the default value is initialized msg_max.
|
|
|
|
|
|
|
|
/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
|
|
|
|
the default message size value if attr parameter of mq_open(2) is NULL. If it
|
|
|
|
exceed msgsize_max, the default value is initialized msgsize_max.
|
2009-04-03 01:57:20 +02:00
|
|
|
|
|
|
|
4. /proc/sys/fs/epoll - Configuration options for the epoll interface
|
|
|
|
--------------------------------------------------------
|
|
|
|
|
|
|
|
This directory contains configuration options for the epoll(7) interface.
|
|
|
|
|
|
|
|
max_user_watches
|
|
|
|
----------------
|
|
|
|
|
|
|
|
Every epoll file descriptor can store a number of files to be monitored
|
|
|
|
for event readiness. Each one of these monitored files constitutes a "watch".
|
|
|
|
This configuration option sets the maximum number of "watches" that are
|
|
|
|
allowed for each user.
|
|
|
|
Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
|
|
|
|
on a 64bit one.
|
|
|
|
The current default value for max_user_watches is the 1/32 of the available
|
|
|
|
low memory, divided for the "watch" cost in bytes.
|
|
|
|
|