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