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Date: Fri, 3 May 2024 00:41:52 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Kees Cook <keescook@chromium.org>
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Subject: Re: [PATCH 5/5] fs: Convert struct file::f_count to refcount_long_t
Message-ID: <20240502234152.GP2118490@ZenIV>
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Sender: Al Viro <viro@ftp.linux.org.uk>

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.