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d29216842a
CAI Qian <caiqian@redhat.com> pointed out that the semantics
of shared subtrees make it possible to create an exponentially
increasing number of mounts in a mount namespace.
mkdir /tmp/1 /tmp/2
mount --make-rshared /
for i in $(seq 1 20) ; do mount --bind /tmp/1 /tmp/2 ; done
Will create create 2^20 or 1048576 mounts, which is a practical problem
as some people have managed to hit this by accident.
As such CVE-2016-6213 was assigned.
Ian Kent <raven@themaw.net> described the situation for autofs users
as follows:
> The number of mounts for direct mount maps is usually not very large because of
> the way they are implemented, large direct mount maps can have performance
> problems. There can be anywhere from a few (likely case a few hundred) to less
> than 10000, plus mounts that have been triggered and not yet expired.
>
> Indirect mounts have one autofs mount at the root plus the number of mounts that
> have been triggered and not yet expired.
>
> The number of autofs indirect map entries can range from a few to the common
> case of several thousand and in rare cases up to between 30000 and 50000. I've
> not heard of people with maps larger than 50000 entries.
>
> The larger the number of map entries the greater the possibility for a large
> number of active mounts so it's not hard to expect cases of a 1000 or somewhat
> more active mounts.
So I am setting the default number of mounts allowed per mount
namespace at 100,000. This is more than enough for any use case I
know of, but small enough to quickly stop an exponential increase
in mounts. Which should be perfect to catch misconfigurations and
malfunctioning programs.
For anyone who needs a higher limit this can be changed by writing
to the new /proc/sys/fs/mount-max sysctl.
Tested-by: CAI Qian <caiqian@redhat.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
331 lines
12 KiB
Plaintext
331 lines
12 KiB
Plaintext
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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(c) 2009, Shen Feng<shen@cn.fujitsu.com>
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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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1. /proc/sys/fs
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----------------------------------------------------------
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Currently, these files are in /proc/sys/fs:
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- aio-max-nr
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- aio-nr
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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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- nr_open
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- overflowuid
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- overflowgid
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- pipe-user-pages-hard
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- pipe-user-pages-soft
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- protected_hardlinks
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- protected_symlinks
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- suid_dumpable
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- super-max
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- super-nr
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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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==============================================================
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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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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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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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==============================================================
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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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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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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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protected_hardlinks:
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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
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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 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
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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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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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privilege levels or is execute only will not be dumped.
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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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This is insecure as it allows regular users to examine the memory
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contents of privileged processes.
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2 - (suidsafe) - any binary which normally would not be dumped is dumped
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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
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to treat privileged core dumps with care, or specific directory
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defined for catching core dumps. If a core dump happens without
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a pipe handler or fully qualifid path, a message will be emitted
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to syslog warning about the lack of a correct setting.
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==============================================================
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super-max & super-nr:
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These numbers control the maximum number of superblocks, and
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thus the maximum number of mounted filesystems the kernel
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can have. You only need to increase super-max if you need to
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mount more filesystems than the current value in super-max
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allows you to.
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==============================================================
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aio-nr & aio-max-nr:
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aio-nr shows the current system-wide number of asynchronous io
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requests. aio-max-nr allows you to change the maximum value
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aio-nr can grow to.
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==============================================================
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mount-max:
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This denotes the maximum number of mounts that may exist
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in a mount namespace.
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==============================================================
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2. /proc/sys/fs/binfmt_misc
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----------------------------------------------------------
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Documentation for the files in /proc/sys/fs/binfmt_misc is
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in Documentation/binfmt_misc.txt.
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3. /proc/sys/fs/mqueue - POSIX message queues filesystem
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----------------------------------------------------------
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The "mqueue" filesystem provides the necessary kernel features to enable the
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creation of a user space library that implements the POSIX message queues
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API (as noted by the MSG tag in the POSIX 1003.1-2001 version of the System
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Interfaces specification.)
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The "mqueue" filesystem contains values for determining/setting the amount of
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resources used by the file system.
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/proc/sys/fs/mqueue/queues_max is a read/write file for setting/getting the
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maximum number of message queues allowed on the system.
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/proc/sys/fs/mqueue/msg_max is a read/write file for setting/getting the
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maximum number of messages in a queue value. In fact it is the limiting value
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for another (user) limit which is set in mq_open invocation. This attribute of
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a queue must be less or equal then msg_max.
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/proc/sys/fs/mqueue/msgsize_max is a read/write file for setting/getting the
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maximum message size value (it is every message queue's attribute set during
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its creation).
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/proc/sys/fs/mqueue/msg_default is a read/write file for setting/getting the
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default number of messages in a queue value if attr parameter of mq_open(2) is
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NULL. If it exceed msg_max, the default value is initialized msg_max.
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/proc/sys/fs/mqueue/msgsize_default is a read/write file for setting/getting
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the default message size value if attr parameter of mq_open(2) is NULL. If it
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exceed msgsize_max, the default value is initialized msgsize_max.
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4. /proc/sys/fs/epoll - Configuration options for the epoll interface
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--------------------------------------------------------
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This directory contains configuration options for the epoll(7) interface.
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max_user_watches
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----------------
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Every epoll file descriptor can store a number of files to be monitored
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for event readiness. Each one of these monitored files constitutes a "watch".
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This configuration option sets the maximum number of "watches" that are
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allowed for each user.
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Each "watch" costs roughly 90 bytes on a 32bit kernel, and roughly 160 bytes
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on a 64bit one.
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The current default value for max_user_watches is the 1/32 of the available
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low memory, divided for the "watch" cost in bytes.
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