Re: [PATCH stable 4.19-6.6] filelock: Remove locks reliably when fcntl/close race is detected

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Jul 23, 2024 at 02:56:08PM +0200, Greg KH wrote:
> On Mon, Jul 22, 2024 at 04:22:50PM +0200, Jann Horn wrote:
> > 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>
> > ---
> >  fs/locks.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/locks.c b/fs/locks.c
> > index fb717dae9029..31659a2d9862 100644
> > --- a/fs/locks.c
> > +++ b/fs/locks.c
> > @@ -2381,8 +2381,9 @@ 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 &&
> > @@ -2397,9 +2398,7 @@ int fcntl_setlk(unsigned int fd, struct file *filp, unsigned int cmd,
> >  		f = files_lookup_fd_locked(files, fd);
> >  		spin_unlock(&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, files);
> 
> Wait, this breaks the build on 5.4.y with the error:
> 
> fs/locks.c: In function ‘fcntl_setlk’:
> fs/locks.c:2545:50: error: ‘files’ undeclared (first use in this function); did you mean ‘file’?
>  2545 |                         locks_remove_posix(filp, files);
>       |                                                  ^~~~~
>       |                                                  file
> 
> I didn't do test-builds yesterday, my fault for not noticing this yet.
> 
> I've dropped this from the 5.4.y queues for now, can you fix this up and send
> an updated version, or give me a hint as to what to do instead?  Odd that this
> works on 4.19.y, let me see why...

Ah, I see why, it applied to the wrong function in 4.19 and that didn't
get built on my test systems (i.e. 64bit only.)  And I see how to fix
this up, let me go do that now, sorry for the noise.

greg k-h