Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t

Kees Cook
2024-05-03T00:10:19Z

On Fri, May 03, 2024 at 12:41:52AM +0100, Al Viro wrote:
> On Thu, May 02, 2024 at 04:21:13PM -0700, Kees Cook wrote:
> > On Fri, May 03, 2024 at 12:12:28AM +0100, Al Viro wrote:
> > > On Thu, May 02, 2024 at 03:52:21PM -0700, Kees Cook wrote:
> > >
> > > > As for semantics, what do you mean? Detecting dec-below-zero means we
> > > > catch underflow, and detected inc-from-zero means we catch resurrection
> > > > attempts. In both cases we avoid double-free, but we have already lost
> > > > to a potential dangling reference to a freed struct file. But just
> > > > letting f_count go bad seems dangerous.
> > >
> > > Detected inc-from-zero can also mean an RCU lookup detecting a descriptor
> > > in the middle of getting closed. And it's more subtle than that, actually,
> > > thanks to SLAB_TYPESAFE_BY_RCU for struct file.
> >
> > But isn't that already handled by __get_file_rcu()? i.e. shouldn't it be
> > impossible for a simple get_file() to ever see a 0 f_count under normal
> > conditions?
>
> For get_file() it is impossible. The comment about semantics had been
> about the sane ways to recover if such crap gets detected.
>
> __get_file_rcu() is a separate story - consider the comment in there:
> * atomic_long_inc_not_zero() above provided a full memory
> * barrier when we acquired a reference.
> *
> * This is paired with the write barrier from assigning to the
> * __rcu protected file pointer so that if that pointer still
> * matches the current file, we know we have successfully
> * acquired a reference to the right file.
>
> and IIRC, refcount_t is weaker wrt barriers.

I think that was also fixed for refcount_t. I'll need to go dig out the
commit...

But anyway, there needs to be a general "oops I hit 0"-aware form of
get_file(), and it seems like it should just be get_file() itself...

--
Kees Cook