fs/namespace.c: WARN if mnt_count has become negative

Browse source

[ Upstream commit edf7ddbf1c ]

Missing calls to mntget() (or equivalently, too many calls to mntput())
are hard to detect because mntput() delays freeing mounts using
task_work_add(), then again using call_rcu().  As a result, mnt_count
can often be decremented to -1 without getting a KASAN use-after-free
report.  Such cases are still bugs though, and they point to real
use-after-frees being possible.

For an example of this, see the bug fixed by commit 1b0b9cc8d3
("vfs: fsmount: add missing mntget()"), discussed at
https://lkml.kernel.org/linux-fsdevel/20190605135401.GB30925@xxxxxxxxxxxxxxxxxxxxxxxxx/T/#u.
This bug *should* have been trivial to find.  But actually, it wasn't
found until syzkaller happened to use fchdir() to manipulate the
reference count just right for the bug to be noticeable.

Address this by making mntput_no_expire() issue a WARN if mnt_count has
become negative.

Suggested-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

...

This commit is contained in:

Eric Biggers 2020-10-31 21:40:21 -07:00 • committed by Greg Kroah-Hartman

parent b1e155ccc8

commit eae1fb3bc5

2 changed files with 7 additions and 4 deletions

Show all changes Ignore whitespace when comparing lines Ignore changes in amount of whitespace Ignore changes in whitespace at EOL

Download patch file Download diff file Expand all files Collapse all files

9

fs/namespace.c

View file

2

fs/pnode.h

View file