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

Kees Cook
2024-05-02T23:21:14Z

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?

--
Kees Cook