linux.git/fs/mount.h, branch v4.9

mnt: Add a per mount namespace limit on the number of mounts

2016-09-30T17:46:48+00:00

CAI Qian  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  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 
Signed-off-by: "Eric W. Biederman"

mntns: Add a limit on the number of mount namespaces.

2016-08-31T12:28:35+00:00

v2: Fixed the very obvious lack of setting ucounts
    on struct mnt_ns reported by Andrei Vagin, and the kbuild
    test report.

Reported-by: Andrei Vagin 
Acked-by: Kees Cook 
Signed-off-by: "Eric W. Biederman"

fs: use seq_open_private() for proc_mounts

2015-07-01T02:44:56+00:00

A patchset to remove support for passing pre-allocated struct seq_file to
seq_open().  Such feature is undocumented and prone to error.

In particular, if seq_release() is used in release handler, it will
kfree() a pointer which was not allocated by seq_open().

So this patchset drops support for pre-allocated struct seq_file: it's
only of use in proc_namespace.c and can be easily replaced by using
seq_open_private()/seq_release_private().

Additionally, it documents the use of file->private_data to hold pointer
to struct seq_file by seq_open().

This patch (of 3):

Since patch described below, from v2.6.15-rc1, seq_open() could use a
struct seq_file already allocated by the caller if the pointer to the
structure is stored in file->private_data before calling the function.

    Commit 1abe77b0fc4b485927f1f798ae81a752677e1d05
    Author: Al Viro 
    Date:   Mon Nov 7 17:15:34 2005 -0500

        [PATCH] allow callers of seq_open do allocation themselves

        Allow caller of seq_open() to kmalloc() seq_file + whatever else they
        want and set ->private_data to it.  seq_open() will then abstain from
        doing allocation itself.

Such behavior is only used by mounts_open_common().

In order to drop support for such uncommon feature, proc_mounts is
converted to use seq_open_private(), which take care of allocating the
proc_mounts structure, making it available through ->private in struct
seq_file.

Conversely, proc_mounts is converted to use seq_release_private(), in
order to release the private structure allocated by seq_open_private().

Then, ->private is used directly instead of proc_mounts() macro to access
to the proc_mounts structure.

Link: http://lkml.kernel.org/r/cover.1433193673.git.ydroneaud@opteya.com
Signed-off-by: Yann Droneaud 
Cc: Al Viro 
Signed-off-by: Andrew Morton 
Signed-off-by: Linus Torvalds

new helper: __legitimize_mnt()

2015-05-11T12:13:14+00:00

same as legitimize_mnt(), except that it does *not* drop and regain
rcu_read_lock; return values are
0  =>  grabbed a reference, we are fine
1  =>  failed, just go away
-1 =>  failed, go away and mntput(bastard) when outside of rcu_read_lock

Signed-off-by: Al Viro

switch the IO-triggering parts of umount to fs_pin

2015-01-26T04:17:29+00:00

Signed-off-by: Al Viro

common object embedded into various struct ....ns

2014-12-04T19:31:00+00:00

for now - just move corresponding ->proc_inum instances over there

Acked-by: "Eric W. Biederman" 
Signed-off-by: Al Viro

vfs: Add a function to lazily unmount all mounts from any dentry.

2014-10-09T06:38:55+00:00

The new function detach_mounts comes in two pieces.  The first piece
is a static inline test of d_mounpoint that returns immediately
without taking any locks if d_mounpoint is not set.  In the common
case when mountpoints are absent this allows the vfs to continue
running with it's same cacheline foot print.

The second piece of detach_mounts __detach_mounts actually does the
work and it assumes that a mountpoint is present so it is slow and
takes namespace_sem for write, and then locks the mount hash (aka
mount_lock) after a struct mountpoint has been found.

With those two locks held each entry on the list of mounts on a
mountpoint is selected and lazily unmounted until all of the mount
have been lazily unmounted.

v7: Wrote a proper change description and removed the changelog
    documenting deleted wrong turns.

Signed-off-by: Eric W. Biederman 
Signed-off-by: Al Viro

vfs: Keep a list of mounts on a mount point

2014-10-09T06:38:54+00:00

To spot any possible problems call BUG if a mountpoint
is put when it's list of mounts is not empty.

AV: use hlist instead of list_head

Reviewed-by: Miklos Szeredi 
Signed-off-by: Eric W. Biederman 
Signed-off-by: Al Viro

vfs: Don't allow overwriting mounts in the current mount namespace

2014-10-09T06:38:54+00:00

In preparation for allowing mountpoints to be renamed and unlinked
in remote filesystems and in other mount namespaces test if on a dentry
there is a mount in the local mount namespace before allowing it to
be renamed or unlinked.

The primary motivation here are old versions of fusermount unmount
which is not safe if the a path can be renamed or unlinked while it is
verifying the mount is safe to unmount.  More recent versions are simpler
and safer by simply using UMOUNT_NOFOLLOW when unmounting a mount
in a directory owned by an arbitrary user.

Miklos Szeredi  reports this is approach is good
enough to remove concerns about new kernels mixed with old versions
of fusermount.

A secondary motivation for restrictions here is that it removing empty
directories that have non-empty mount points on them appears to
violate the rule that rmdir can not remove empty directories.  As
Linus Torvalds pointed out this is useful for programs (like git) that
test if a directory is empty with rmdir.

Therefore this patch arranges to enforce the existing mount point
semantics for local mount namespace.

v2: Rewrote the test to be a drop in replacement for d_mountpoint
v3: Use bool instead of int as the return type of is_local_mountpoint

Reviewed-by: Miklos Szeredi 
Signed-off-by: "Eric W. Biederman" 
Signed-off-by: Al Viro

delayed mntput

2014-10-09T06:38:53+00:00

On final mntput() we want fs shutdown to happen before return to
userland; however, the only case where we want it happen right
there (i.e. where task_work_add won't do) is MNT_INTERNAL victim.
Those have to be fully synchronous - failure halfway through module
init might count on having vfsmount killed right there.  Fortunately,
final mntput on MNT_INTERNAL vfsmounts happens on shallow stack.
So we handle those synchronously and do an analog of delayed fput
logics for everything else.

As the result, we are guaranteed that fs shutdown will always happen
on shallow stack.

Signed-off-by: Al Viro