Re: [PATCH v2] keys: Fix UAF in key_put()

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Mar 20, 2025 at 07:28:36PM +0200, Jarkko Sakkinen wrote:
> On Thu, Mar 20, 2025 at 04:39:11PM +0000, David Howells wrote:
> > Jarkko Sakkinen <jarkko@kernel.org> wrote:
> > 
> > > > +		if (test_bit(KEY_FLAG_FINAL_PUT, &key->flags)) {
> > > > +			smp_mb(); /* Clobber key->user after FINAL_PUT seen. */
> > > 
> > > test_bit() is already atomic.
> > 
> > Atomiticity doesn't apply to test_bit() - it only matters when it does two (or
> > more) accesses that must be perceptually indivisible (e.g. set_bit doing RMW).
> > 
> > But atomiticity isn't the issue here, hence the barrier.  You need to be
> > looking at memory-barriers.txt, not atomic_bitops.txt.
> > 
> > We have two things to correctly order and set_bit() does not imply sufficient
> > barriering; test_and_set_bit() does, but not set_bit(), hence Linus's comment
> > about really wanting a set_bit_release().
> 
> Oops, I was hallucinating here. And yeah, test_and_set_bit() does
> imply full mb as you said.
> 
> I was somehow remembering what I did in SGX driver incorrectly and
> that led me into misconclusions, sorry.
> 
> if (test_and_set_bit(SGX_ENCL_IOCTL, &encl->flags))
> 	return -EBUSY;
> 
> > 
> > > > +			smp_mb(); /* key->user before FINAL_PUT set. */
> > > > +			set_bit(KEY_FLAG_FINAL_PUT, &key->flags);
> > > 
> > > Ditto.
> > 
> > Ditto. ;-)
> 
> Duh, no need poke with the stick further (or deeper) ;-)
> 
> > 
> > > Nit: I'm just thinking should the name imply more like that "now
> > > key_put() is actually done". E.g., even something like KEY_FLAG_PUT_DONE
> > > would be more self-descriptive.
> > 
> > KEY_FLAG_PUT_DONE isn't right.  There can be lots of puts on a single key -
> > only the one that reduces it to 0 matters for this.  You could call it
> > KEY_FLAG_CAN_NOW_GC or KEY_FLAG_GC_ABLE.
> 
> Well all alternatives are fine but my thinking was that one that finally
> zeros the refcount, "finalizes put" (pick whatever you want anyway).

I'll pick this one up tomorrow and put a PR out within this week.

BR, Jarkko