[PATCH v2] locks: Fix dropped call to ->fl_release_private()

[Jeff Layton]

On Wed, 2022-08-17 at 19:41 +0100, David Howells wrote:
> 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 at amazon.com>
> cc: Chuck Lever <chuck.lever at oracle.com>
> cc: Jeff Layton <jlayton at kernel.org>
> cc: Marc Dionne <marc.dionne at auristor.com>
> cc: linux-afs at lists.infradead.org
> cc: linux-fsdevel at 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);
>  
> 
> 

Looks good. I'll get this into -next and plan to get it up to Linus
soon.

Thanks!
-- 
Jeff Layton <jlayton at kernel.org>