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

[Iwashima, Kuniyuki]

From: Jeff Layton <jlayton@xxxxxxxxxx>
Date: Wed, 17 Aug 2022 14:42:57 -0400
> 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@xxxxxxxxxx>
> > cc: Chuck Lever <chuck.lever@xxxxxxxxxx>
> > cc: Jeff Layton <jlayton@xxxxxxxxxx>
> > cc: Marc Dionne <marc.dionne@xxxxxxxxxxxx>
> > cc: linux-afs@xxxxxxxxxxxxxxxxxxx
> > cc: linux-fsdevel@xxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/r/166075758809.3532462.13307935588777587536.stgit@xxxxxxxxxxxxxxxxxxxxxx/ # v1

nit: missing Signed-off-by :)

Acked-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>

Thanks for the fix!


> > ---
> >
> >  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@xxxxxxxxxx>
>