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virtiofsd: stay below fs.file-max sysctl value (CVE-2020-10717) 

The system-wide fs.file-max sysctl value determines how many files can
be open.  It defaults to a value calculated based on the machine's RAM
size.  Previously virtiofsd would try to set RLIMIT_NOFILE to 1,000,000
and this allowed the FUSE client to exhaust the number of open files
system-wide on Linux hosts with less than 10 GB of RAM!

Take fs.file-max into account when choosing the default RLIMIT_NOFILE
value.

Fixes: CVE-2020-10717
Reported-by: Yuval Avrahami <yavrahami@paloaltonetworks.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20200501140644.220940-3-stefanha@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-05-01 15:06:44 +01:00 committed by Dr. David Alan Gilbert
parent 6dbb716877
commit 8c1d353d10
1 changed files with 25 additions and 1 deletions
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26
tools/virtiofsd/helper.c
View File

@ -176,7 +176,8 @@ void fuse_cmdline_help(void)
" default: no_xattr\n"
" --rlimit-nofile=<num> set maximum number of file descriptors\n"
" (0 leaves rlimit unchanged)\n"
" default: 1,000,000 if the current rlimit is lower\n"
" default: min(1000000, fs.file-max - 16384)\n"
" if the current rlimit is lower\n"
);
}
@ -199,9 +200,32 @@ static int fuse_helper_opt_proc(void *data, const char *arg, int key,
static unsigned long get_default_rlimit_nofile(void)
{
g_autofree gchar *file_max_str = NULL;
const rlim_t reserved_fds = 16384; /* leave at least this many fds free */
rlim_t max_fds = 1000000; /* our default RLIMIT_NOFILE target */
rlim_t file_max;
struct rlimit rlim;
/*
* Reduce max_fds below the system-wide maximum, if necessary. This
* ensures there are fds available for other processes so we don't
* cause resource exhaustion.
*/
if (!g_file_get_contents("/proc/sys/fs/file-max", &file_max_str,
NULL, NULL)) {
fuse_log(FUSE_LOG_ERR, "can't read /proc/sys/fs/file-max\n");
exit(1);
}
file_max = g_ascii_strtoull(file_max_str, NULL, 10);
if (file_max < 2 * reserved_fds) {
fuse_log(FUSE_LOG_ERR,
"The fs.file-max sysctl is too low (%lu) to allow a "
"reasonable number of open files.\n",
(unsigned long)file_max);
exit(1);
}
max_fds = MIN(file_max - reserved_fds, max_fds);
if (getrlimit(RLIMIT_NOFILE, &rlim) < 0) {
fuse_log(FUSE_LOG_ERR, "getrlimit(RLIMIT_NOFILE): %m\n");
exit(1);