Re: [PATCH v2 1/1] cdrom: Fix spectre-v1 gadget

[Phillip Potter <phil@philpotter.co.uk>]

On Thu, Jun 15, 2023 at 08:14:47PM -0700, Pawan Gupta wrote:
> On Fri, Jun 16, 2023 at 12:31:50AM +0100, Phillip Potter wrote:
> > I've now looked at this. It is possible for cdi->capacity to be > 1, as
> > it is set via get_capabilities() -> cdrom_number_of_slots(), if the
> > device is an individual or cartridge changer.
> 
> Ohk. Is there an upper limit to cdi->capacity? If not, we are left with
> barrier_nospec().
> 

No, from the perspective of the codebase, this value is read from the
device and is therefore arbitrary in theory.

> > Therefore, I think using CDI_MAX_CAPACITY of 1 is not the correct
> > approach. Jordy's V2 patch is fine therefore, but perhaps using
> > array_index_nospec() with cdi->capacity is still better than a
> > do/while loop from a performance perspective, given it would be cached
> > etc. at that point, so possibly quicker. Thoughts? (I'm no expert on
> > spectre-v1 I'll admit).
> 
> array_index_nospec() can only clip the arg correctly if the upper bound
> is correct. Problem with array_index_nospec(arg, cdi->capacity) is
> cdi->capacity is not a constant, so it suffers from the same problem as
> arg i.e. cdi->capacity could also be speculated. Although having to
> control 2 loads makes the attack difficult, but does not rules out
> completely.
> 
> barrier_nospec() makes the CPU wait for all previous loads to retire
> before executing following instructions speculatively. This causes the
> conditional branch to resolve correctly. I hope this does not fall into
> a hotpath.

Thanks for the explanation. Based on this and the fact that particular
ioctl function isn't likely on a hugely hot path for most users, I have
approved the patch via another e-mail.

Regards,
Phil