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>
330 lines
12 KiB
Text
330 lines
12 KiB
Text
Documentation for /proc/sys/fs/* kernel version 2.2.10
|
|
(c) 1998, 1999, Rik van Riel <riel@nl.linux.org>
|
|
(c) 2009, Shen Feng<shen@cn.fujitsu.com>
|
|
|
|
For general info and legal blurb, please look in README.
|
|
|
|
==============================================================
|
|
|
|
This file contains documentation for the sysctl files in
|
|
/proc/sys/fs/ and is valid for Linux kernel version 2.2.
|
|
|
|
The files in this directory can be used to tune and monitor
|
|
miscellaneous and general things in the operation of the Linux
|
|
kernel. Since some of the files _can_ be used to screw up your
|
|
system, it is advisable to read both documentation and source
|
|
before actually making adjustments.
|
|
|
|
1. /proc/sys/fs
|
|
----------------------------------------------------------
|
|
|
|
Currently, these files are in /proc/sys/fs:
|
|
- aio-max-nr
|
|
- aio-nr
|
|
- dentry-state
|
|
- dquot-max
|
|
- dquot-nr
|
|
- file-max
|
|
- file-nr
|
|
- inode-max
|
|
- inode-nr
|
|
- inode-state
|
|
- nr_open
|
|
- overflowuid
|
|
- overflowgid
|
|
- pipe-user-pages-hard
|
|
- pipe-user-pages-soft
|
|
- protected_hardlinks
|
|
- protected_symlinks
|
|
- suid_dumpable
|
|
- super-max
|
|
- super-nr
|
|
|
|
==============================================================
|
|
|
|
aio-nr & aio-max-nr:
|
|
|
|
aio-nr is the running total of the number of events specified on the
|
|
io_setup system call for all currently active aio contexts. If aio-nr
|
|
reaches aio-max-nr then io_setup will fail with EAGAIN. Note that
|
|
raising aio-max-nr does not result in the pre-allocation or re-sizing
|
|
of any kernel data structures.
|
|
|
|
==============================================================
|
|
|
|
dentry-state:
|
|
|
|
From linux/fs/dentry.c:
|
|
--------------------------------------------------------------
|
|
struct {
|
|
int nr_dentry;
|
|
int nr_unused;
|
|
int age_limit; /* age in seconds */
|
|
int want_pages; /* pages requested by system */
|
|
int dummy[2];
|
|
} dentry_stat = {0, 0, 45, 0,};
|
|
--------------------------------------------------------------
|
|
|
|
Dentries are dynamically allocated and deallocated, and
|
|
nr_dentry seems to be 0 all the time. Hence it's safe to
|
|
assume that only nr_unused, age_limit and want_pages are
|
|
used. Nr_unused seems to be exactly what its name says.
|
|
Age_limit is the age in seconds after which dcache entries
|
|
can be reclaimed when memory is short and want_pages is
|
|
nonzero when shrink_dcache_pages() has been called and the
|
|
dcache isn't pruned yet.
|
|
|
|
==============================================================
|
|
|
|
dquot-max & dquot-nr:
|
|
|
|
The file dquot-max shows the maximum number of cached disk
|
|
quota entries.
|
|
|
|
The file dquot-nr shows the number of allocated disk quota
|
|
entries and the number of free disk quota entries.
|
|
|
|
If the number of free cached disk quotas is very low and
|
|
you have some awesome number of simultaneous system users,
|
|
you might want to raise the limit.
|
|
|
|
==============================================================
|
|
|
|
file-max & file-nr:
|
|
|
|
The value in file-max denotes the maximum number of file-
|
|
handles that the Linux kernel will allocate. When you get lots
|
|
of error messages about running out of file handles, you might
|
|
want to increase this limit.
|
|
|
|
Historically,the kernel was able to allocate file handles
|
|
dynamically, but not to free them again. The three values in
|
|
file-nr denote the number of allocated file handles, the number
|
|
of allocated but unused file handles, and the maximum number of
|
|
file handles. Linux 2.6 always reports 0 as the number of free
|
|
file handles -- this is not an error, it just means that the
|
|
number of allocated file handles exactly matches the number of
|
|
used file handles.
|
|
|
|
Attempts to allocate more file descriptors than file-max are
|
|
reported with printk, look for "VFS: file-max limit <number>
|
|
reached".
|
|
==============================================================
|
|
|
|
nr_open:
|
|
|
|
This denotes the maximum number of file-handles a process can
|
|
allocate. Default value is 1024*1024 (1048576) which should be
|
|
enough for most machines. Actual limit depends on RLIMIT_NOFILE
|
|
resource limit.
|
|
|
|
==============================================================
|
|
|
|
inode-max, inode-nr & inode-state:
|
|
|
|
As with file handles, the kernel allocates the inode structures
|
|
dynamically, but can't free them yet.
|
|
|
|
The value in inode-max denotes the maximum number of inode
|
|
handlers. This value should be 3-4 times larger than the value
|
|
in file-max, since stdin, stdout and network sockets also
|
|
need an inode struct to handle them. When you regularly run
|
|
out of inodes, you need to increase this value.
|
|
|
|
The file inode-nr contains the first two items from
|
|
inode-state, so we'll skip to that file...
|
|
|
|
Inode-state contains three actual numbers and four dummies.
|
|
The actual numbers are, in order of appearance, nr_inodes,
|
|
nr_free_inodes and preshrink.
|
|
|
|
Nr_inodes stands for the number of inodes the system has
|
|
allocated, this can be slightly more than inode-max because
|
|
Linux allocates them one pageful at a time.
|
|
|
|
Nr_free_inodes represents the number of free inodes (?) and
|
|
preshrink is nonzero when the nr_inodes > inode-max and the
|
|
system needs to prune the inode list instead of allocating
|
|
more.
|
|
|
|
==============================================================
|
|
|
|
overflowgid & overflowuid:
|
|
|
|
Some filesystems only support 16-bit UIDs and GIDs, although in Linux
|
|
UIDs and GIDs are 32 bits. When one of these filesystems is mounted
|
|
with writes enabled, any UID or GID that would exceed 65535 is translated
|
|
to a fixed value before being written to disk.
|
|
|
|
These sysctls allow you to change the value of the fixed UID and GID.
|
|
The default is 65534.
|
|
|
|
==============================================================
|
|
|
|
pipe-user-pages-hard:
|
|
|
|
Maximum total number of pages a non-privileged user may allocate for pipes.
|
|
Once this limit is reached, no new pipes may be allocated until usage goes
|
|
below the limit again. When set to 0, no limit is applied, which is the default
|
|
setting.
|
|
|
|
==============================================================
|
|
|
|
pipe-user-pages-soft:
|
|
|
|
Maximum total number of pages a non-privileged user may allocate for pipes
|
|
before the pipe size gets limited to a single page. Once this limit is reached,
|
|
new pipes will be limited to a single page in size for this user in order to
|
|
limit total memory usage, and trying to increase them using fcntl() will be
|
|
denied until usage goes below the limit again. The default value allows to
|
|
allocate up to 1024 pipes at their default size. When set to 0, no limit is
|
|
applied.
|
|
|
|
==============================================================
|
|
|
|
protected_hardlinks:
|
|
|
|
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,
|
|
on systems without separated partitions, this stops unauthorized users
|
|
from "pinning" vulnerable setuid/setgid files against being upgraded by
|
|
the administrator, or linking to special files.
|
|
|
|
When set to "0", hardlink creation behavior is unrestricted.
|
|
|
|
When set to "1" hardlinks cannot be created by users if they do not
|
|
already own the source file, or do not have read/write access to it.
|
|
|
|
This protection is based on the restrictions in Openwall and grsecurity.
|
|
|
|
==============================================================
|
|
|
|
protected_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
|
|
|
|
When set to "0", symlink following behavior is unrestricted.
|
|
|
|
When set to "1" symlinks are permitted to be followed only 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.
|
|
|
|
This protection is based on the restrictions in Openwall and grsecurity.
|
|
|
|
==============================================================
|
|
|
|
suid_dumpable:
|
|
|
|
This value can be used to query and set the core dump mode for setuid
|
|
or otherwise protected/tainted binaries. The modes are
|
|
|
|
0 - (default) - traditional behaviour. Any process which has changed
|
|
privilege levels or is execute only will not be dumped.
|
|
1 - (debug) - all processes dump core when possible. The core dump is
|
|
owned by the current user and no security is applied. This is
|
|
intended for system debugging situations only. Ptrace is unchecked.
|
|
This is insecure as it allows regular users to examine the memory
|
|
contents of privileged processes.
|
|
2 - (suidsafe) - any binary which normally would not be dumped is dumped
|
|
anyway, but only if the "core_pattern" kernel sysctl is set to
|
|
either a pipe handler or a fully qualified path. (For more details
|
|
on this limitation, see CVE-2006-2451.) This mode is appropriate
|
|
when administrators are attempting to debug problems in a normal
|
|
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.
|
|
|
|
==============================================================
|
|
|
|
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.
|
|
|
|
==============================================================
|
|
|
|
mount-max:
|
|
|
|
This denotes the maximum number of mounts that may exist
|
|
in a mount namespace.
|
|
|
|
==============================================================
|
|
|
|
|
|
2. /proc/sys/fs/binfmt_misc
|
|
----------------------------------------------------------
|
|
|
|
Documentation for the files in /proc/sys/fs/binfmt_misc is
|
|
in Documentation/binfmt_misc.txt.
|
|
|
|
|
|
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).
|
|
|
|
/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.
|
|
|
|
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.
|
|
|