Commit Graph

13 Commits

Author SHA1 Message Date
Joe Lawrence 93862e385d livepatch: add (un)patch callbacks
Provide livepatch modules a klp_object (un)patching notification
mechanism.  Pre and post-(un)patch callbacks allow livepatch modules to
setup or synchronize changes that would be difficult to support in only
patched-or-unpatched code contexts.

Callbacks can be registered for target module or vmlinux klp_objects,
but each implementation is klp_object specific.

  - Pre-(un)patch callbacks run before any (un)patching transition
    starts.

  - Post-(un)patch callbacks run once an object has been (un)patched and
    the klp_patch fully transitioned to its target state.

Example use cases include modification of global data and registration
of newly available services/handlers.

See Documentation/livepatch/callbacks.txt for details and
samples/livepatch/ for examples.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2017-10-19 10:08:56 +02:00
Petr Mladek 19205da6a0 livepatch: Small shadow variable documentation fixes
The description of the basic operations was a bit inconsistent
and based on older version of the patchset.

Also the size of the spinlock structure should be allocated
instead of the pointer.

Signed-off-by: Petr Mladek <pmladek@suse.com>
Acked-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2017-10-02 11:55:24 +02:00
Joe Lawrence 439e7271dc livepatch: introduce shadow variable API
Add exported API for livepatch modules:

  klp_shadow_get()
  klp_shadow_alloc()
  klp_shadow_get_or_alloc()
  klp_shadow_free()
  klp_shadow_free_all()

that implement "shadow" variables, which allow callers to associate new
shadow fields to existing data structures.  This is intended to be used
by livepatch modules seeking to emulate additions to data structure
definitions.

See Documentation/livepatch/shadow-vars.txt for a summary of the new
shadow variable API, including a few common use cases.

See samples/livepatch/livepatch-shadow-* for example modules that
demonstrate shadow variables.

[jkosina@suse.cz: fix __klp_shadow_get_or_alloc() comment as spotted by
 Josh]
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2017-09-14 23:06:12 +02:00
Josh Poimboeuf 3ec24776bf livepatch: allow removal of a disabled patch
Currently we do not allow patch module to unload since there is no
method to determine if a task is still running in the patched code.

The consistency model gives us the way because when the unpatching
finishes we know that all tasks were marked as safe to call an original
function. Thus every new call to the function calls the original code
and at the same time no task can be somewhere in the patched code,
because it had to leave that code to be marked as safe.

We can safely let the patch module go after that.

Completion is used for synchronization between module removal and sysfs
infrastructure in a similar way to commit 942e443127 ("module: Fix
mod->mkobj.kobj potentially freed too early").

Note that we still do not allow the removal for immediate model, that is
no consistency model. The module refcount may increase in this case if
somebody disables and enables the patch several times. This should not
cause any harm.

With this change a call to try_module_get() is moved to
__klp_enable_patch from klp_register_patch to make module reference
counting symmetric (module_put() is in a patch disable path) and to
allow to take a new reference to a disabled module when being enabled.

Finally, we need to be very careful about possible races between
klp_unregister_patch(), kobject_put() functions and operations
on the related sysfs files.

kobject_put(&patch->kobj) must be called without klp_mutex. Otherwise,
it might be blocked by enabled_store() that needs the mutex as well.
In addition, enabled_store() must check if the patch was not
unregisted in the meantime.

There is no need to do the same for other kobject_put() callsites
at the moment. Their sysfs operations neither take the lock nor
they access any data that might be freed in the meantime.

There was an attempt to use kobjects the right way and prevent these
races by design. But it made the patch definition more complicated
and opened another can of worms. See
https://lkml.kernel.org/r/1464018848-4303-1-git-send-email-pmladek@suse.com

[Thanks to Petr Mladek for improving the commit message.]

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2017-03-08 09:38:43 +01:00
Josh Poimboeuf d83a7cb375 livepatch: change to a per-task consistency model
Change livepatch to use a basic per-task consistency model.  This is the
foundation which will eventually enable us to patch those ~10% of
security patches which change function or data semantics.  This is the
biggest remaining piece needed to make livepatch more generally useful.

This code stems from the design proposal made by Vojtech [1] in November
2014.  It's a hybrid of kGraft and kpatch: it uses kGraft's per-task
consistency and syscall barrier switching combined with kpatch's stack
trace switching.  There are also a number of fallback options which make
it quite flexible.

Patches are applied on a per-task basis, when the task is deemed safe to
switch over.  When a patch is enabled, livepatch enters into a
transition state where tasks are converging to the patched state.
Usually this transition state can complete in a few seconds.  The same
sequence occurs when a patch is disabled, except the tasks converge from
the patched state to the unpatched state.

An interrupt handler inherits the patched state of the task it
interrupts.  The same is true for forked tasks: the child inherits the
patched state of the parent.

Livepatch uses several complementary approaches to determine when it's
safe to patch tasks:

1. The first and most effective approach is stack checking of sleeping
   tasks.  If no affected functions are on the stack of a given task,
   the task is patched.  In most cases this will patch most or all of
   the tasks on the first try.  Otherwise it'll keep trying
   periodically.  This option is only available if the architecture has
   reliable stacks (HAVE_RELIABLE_STACKTRACE).

2. The second approach, if needed, is kernel exit switching.  A
   task is switched when it returns to user space from a system call, a
   user space IRQ, or a signal.  It's useful in the following cases:

   a) Patching I/O-bound user tasks which are sleeping on an affected
      function.  In this case you have to send SIGSTOP and SIGCONT to
      force it to exit the kernel and be patched.
   b) Patching CPU-bound user tasks.  If the task is highly CPU-bound
      then it will get patched the next time it gets interrupted by an
      IRQ.
   c) In the future it could be useful for applying patches for
      architectures which don't yet have HAVE_RELIABLE_STACKTRACE.  In
      this case you would have to signal most of the tasks on the
      system.  However this isn't supported yet because there's
      currently no way to patch kthreads without
      HAVE_RELIABLE_STACKTRACE.

3. For idle "swapper" tasks, since they don't ever exit the kernel, they
   instead have a klp_update_patch_state() call in the idle loop which
   allows them to be patched before the CPU enters the idle state.

   (Note there's not yet such an approach for kthreads.)

All the above approaches may be skipped by setting the 'immediate' flag
in the 'klp_patch' struct, which will disable per-task consistency and
patch all tasks immediately.  This can be useful if the patch doesn't
change any function or data semantics.  Note that, even with this flag
set, it's possible that some tasks may still be running with an old
version of the function, until that function returns.

There's also an 'immediate' flag in the 'klp_func' struct which allows
you to specify that certain functions in the patch can be applied
without per-task consistency.  This might be useful if you want to patch
a common function like schedule(), and the function change doesn't need
consistency but the rest of the patch does.

For architectures which don't have HAVE_RELIABLE_STACKTRACE, the user
must set patch->immediate which causes all tasks to be patched
immediately.  This option should be used with care, only when the patch
doesn't change any function or data semantics.

In the future, architectures which don't have HAVE_RELIABLE_STACKTRACE
may be allowed to use per-task consistency if we can come up with
another way to patch kthreads.

The /sys/kernel/livepatch/<patch>/transition file shows whether a patch
is in transition.  Only a single patch (the topmost patch on the stack)
can be in transition at a given time.  A patch can remain in transition
indefinitely, if any of the tasks are stuck in the initial patch state.

A transition can be reversed and effectively canceled by writing the
opposite value to the /sys/kernel/livepatch/<patch>/enabled file while
the transition is in progress.  Then all the tasks will attempt to
converge back to the original patch state.

[1] https://lkml.kernel.org/r/20141107140458.GA21774@suse.cz

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Ingo Molnar <mingo@kernel.org>        # for the scheduler changes
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2017-03-08 09:36:21 +01:00
Linus Torvalds c1aac62f36 A slightly quieter cycle for documentation this time around.
Three more DocBook template files have been converted to RST; only 21 to
 go.  There are various build improvements and the usual array of
 documentation improvements and fixes.
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Merge tag 'docs-4.11' of git://git.lwn.net/linux

Pull documentation updates from Jonathan Corbet:
 "A slightly quieter cycle for documentation this time around.

  Three more DocBook template files have been converted to RST; only 21
  to go. There are various build improvements and the usual array of
  documentation improvements and fixes"

* tag 'docs-4.11' of git://git.lwn.net/linux: (44 commits)
  docs / driver-api: Fix structure references in device_link.rst
  PM / docs: Fix structure references in device.rst
  Add a target to check broken external links in the Documentation
  Documentation: Fix linux-api list typo
  Documentation: DocBook/Makefile comment typo
  Improve sparse documentation
  Documentation: make Makefile.sphinx no-ops quieter
  Documentation: DMA-ISA-LPC.txt
  Documentation: input: fix path to input code definitions
  docs: Remove the copyright year from conf.py
  docs: Fix a warning in the Korean HOWTO.rst translation
  PM / sleep / docs: Convert PM notifiers document to reST
  PM / core / docs: Convert sleep states API document to reST
  PM / core: Update kerneldoc comments in pm.h
  doc-rst: Fix recursive make invocation from macros
  doc-rst: Delete output of failed dot-SVG conversion
  doc-rst: Break shell command sequences on failure
  Documentation/sphinx: make targets independent of Sphinx work for HAVE_SPHINX=0
  doc-rst: fixed cleandoc target when used with O=dir
  Documentation/sphinx: prevent generation of .pyc files in the source tree
  ...
2017-02-22 18:51:29 -08:00
Masanari Iida 8da9704c8b Doc: Fix double words in Documentation
This patch fix some double words found in Documentation.

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
2017-01-26 15:25:41 -07:00
Miroslav Benes 372e2db721 livepatch: doc: remove the limitation for schedule() patching
The Limitations section of the documentation describes the impossibility
to livepatch anything that is inlined to __schedule() function. This had
been true till 4.9 kernel came. Thanks to commit 0100301bfd
("sched/x86: Rewrite the switch_to() code") from Brian Gerst there is
__switch_to_asm function now (implemented in assembly) called properly
from context_switch(). RIP is thus saved on the stack and a task would
return to proper version of __schedule() et al. functions.

Of course __switch_to_asm() is not patchable for the reason described in
the section. But there is no __fentry__ call and I cannot imagine a
reason to do it anyway.

Therefore, remove the paragraphs from the section.

Signed-off-by: Miroslav Benes <mbenes@suse.cz>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2017-01-11 02:33:39 +01:00
Petr Mladek 59024954a1 Documentation/livepatch: Fix stale link to gmame
gmame archive does not longer exist. Use the message id and
generic redirector instead.

Reported-by: John Donnelly <john.donnelly@canonical.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2016-12-09 13:41:46 +01:00
Jessica Yu 5ad75fcdd7 Documentation: livepatch: add section about arch-specific code
Document usage of arch-specific elf sections in livepatch as well
as implementation of arch-specific code.

[jkosina@suse.cz: fix wording as suggested by Petr Mladek]
Signed-off-by: Jessica Yu <jeyu@redhat.com>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2016-08-18 23:41:56 +02:00
Jiri Kosina be69f70e63 Merge branches 'for-4.7/core', 'for-4.7/livepatching-doc' and 'for-4.7/livepatching-ppc64' into for-linus 2016-05-17 12:06:35 +02:00
Petr Mladek 5e4e38446a livepatch: Add some basic livepatch documentation
livepatch framework deserves some documentation, definitely.
This is an attempt to provide some basic info. I hope that
it will be useful for both LivePatch producers and also
potential developers of the framework itself.

[jkosina@suse.cz:
 - incorporated feedback (grammar fixes) from
   Chris J Arges <chris.j.arges@canonical.com>
 - s/LivePatch/livepatch in changelog as pointed out by
   Josh Poimboeuf <jpoimboe@redhat.com>
 - incorporated part of feedback (grammar fixes / reformulations) from
   Balbir Singh <bsingharora@gmail.com>
]

Acked-by: Jessica Yu <jeyu@redhat.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2016-04-27 22:01:32 +02:00
Jessica Yu 3b812ecce7 Documentation: livepatch: outline Elf format and requirements for patch modules
Document livepatch module requirements and the special Elf constants patch
modules use.

Signed-off-by: Jessica Yu <jeyu@redhat.com>
Acked-by: Miroslav Benes <mbenes@suse.cz>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
2016-04-01 15:00:12 +02:00