filelock: Remove locks reliably when fcntl/close race is detected

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@google.com
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>

diff --git a/fs/locks.c b/fs/locks.c
index 28270e74be3428be2b518d4c26daa04e353cff39..7957a5aa84967eabe1b408bdcc57d5b2553077b5 100644 (file)
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -2297,8 +2297,9 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
        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 file *filp, unsigned int cmd,
                f = fcheck(fd);
                spin_unlock(&current->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, &current->files);
                        error = -EBADF;
                }
        }