4.19-stable review patch. If anyone has any objections, please let me know.
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From: Jann Horn jannh@google.com
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@kernel.org Link: https://bugs.chromium.org/p/project-zero/issues/detail?id=2563 Signed-off-by: Jann Horn jannh@google.com Link: https://lore.kernel.org/r/20240702-fs-lock-recover-2-v1-1-edd456f63789@googl... Reviewed-by: Jeff Layton jlayton@kernel.org Signed-off-by: Christian Brauner brauner@kernel.org [stable fixup: ->c.flc_type was ->fl_type in older kernels] Signed-off-by: Jann Horn jannh@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- 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; } }