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.
