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Subject[PATCH v2] locks: Fix dropped call to ->fl_release_private()
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Prior to commit 4149be7bda7e, sys_flock() would allocate the file_lock
struct it was going to use to pass parameters, call ->flock() and then call
locks_free_lock() to get rid of it - which had the side effect of calling
locks_release_private() and thus ->fl_release_private().

With commit 4149be7bda7e, however, this is no longer the case: the struct
is now allocated on the stack, and locks_free_lock() is no longer called -
and thus any remaining private data doesn't get cleaned up either.

This causes afs flock to cause oops. Kasan catches this as a UAF by the
list_del_init() in afs_fl_release_private() for the file_lock record
produced by afs_fl_copy_lock() as the original record didn't get delisted.
It can be reproduced using the generic/504 xfstest.

Fix this by reinstating the locks_release_private() call in sys_flock().
I'm not sure if this would affect any other filesystems. If not, then the
release could be done in afs_flock() instead.

Changes
=======
ver #2)
- Don't need to call ->fl_release_private() after calling the security
hook, only after calling ->flock().

Fixes: 4149be7bda7e ("fs/lock: Don't allocate file_lock in flock_make_lock().")
cc: Kuniyuki Iwashima <kuniyu@amazon.com>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: linux-fsdevel@vger.kernel.org
Link: https://lore.kernel.org/r/166075758809.3532462.13307935588777587536.stgit@warthog.procyon.org.uk/ # v1
---

fs/locks.c | 1 +
1 file changed, 1 insertion(+)

diff --git a/fs/locks.c b/fs/locks.c
index c266cfdc3291..607f94a0e789 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -2129,6 +2129,7 @@ SYSCALL_DEFINE2(flock, unsigned int, fd, unsigned int, cmd)
else
error = locks_lock_file_wait(f.file, &fl);

+ locks_release_private(&fl);
out_putf:
fdput(f);


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