4.19-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jann Horn <jannh@xxxxxxxxxx>
commit 3cad1bc010416c6dd780643476bc59ed742436b9 upstream.
When fcntl_setlk() races with close(), it removes the created lock with
do_lock_file_wait().
However, LSMs can allow the first do_lock_file_wait() that created the lock
while denying the second do_lock_file_wait() that tries to remove the lock.
In theory (but AFAIK not in practice), posix_lock_file() could also fail to
remove a lock due to GFP_KERNEL allocation failure (when splitting a range
in the middle).
After the bug has been triggered, use-after-free reads will occur in
lock_get_status() when userspace reads /proc/locks. This can likely be used
to read arbitrary kernel memory, but can't corrupt kernel memory.
This only affects systems with SELinux / Smack / AppArmor / BPF-LSM in
enforcing mode and only works from some security contexts.
Fix it by calling locks_remove_posix() instead, which is designed to
reliably get rid of POSIX locks associated with the given file and
files_struct and is also used by filp_flush().
Fixes: c293621bbf67 ("[PATCH] stale POSIX lock handling")
Cc: stable@xxxxxxxxxx
Link: https://bugs.chromium.org/p/project-zero/issues/detail?id=2563
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20240702-fs-lock-recover-2-v1-1-edd456f63789@xxxxxxxxxx
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
[stable fixup: ->c.flc_type was ->fl_type in older kernels]
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
fs/locks.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -2297,8 +2297,9 @@ int fcntl_setlk(unsigned int fd, struct
error = do_lock_file_wait(filp, cmd, file_lock);
/*
- * Attempt to detect a close/fcntl race and recover by releasing the
- * lock that was just acquired. There is no need to do that when we're
+ * Detect close/fcntl races and recover by zapping all POSIX locks
+ * associated with this file and our files_struct, just like on
+ * filp_flush(). There is no need to do that when we're
* unlocking though, or for OFD locks.
*/
if (!error && file_lock->fl_type != F_UNLCK &&
@@ -2312,9 +2313,7 @@ int fcntl_setlk(unsigned int fd, struct
f = fcheck(fd);
spin_unlock(¤t->files->file_lock);
if (f != filp) {
- file_lock->fl_type = F_UNLCK;
- error = do_lock_file_wait(filp, cmd, file_lock);
- WARN_ON_ONCE(error);
+ locks_remove_posix(filp, ¤t->files);
error = -EBADF;
}
}
