Re: [v2 PATCH] iommu/arm-smmu-v3: Fix L1 stream table index calculation for 32-bit sid size

[Yang Shi]

On 10/2/24 12:40 PM, Jason Gunthorpe wrote:
> On Wed, Oct 02, 2024 at 12:22:48PM -0700, Nicolin Chen wrote:
> > On Wed, Oct 02, 2024 at 12:04:32PM -0700, Yang Shi wrote:
> > > > On Wed, Oct 02, 2024 at 10:55:14AM -0700, Yang Shi wrote:
> > > > > +static inline unsigned int arm_smmu_strtab_max_sid(struct arm_smmu_device *smmu)
> > > > > +{
> > > > > +       return (1ULL << smmu->sid_bits);
> > > > > +}
> > > > > +
> > > > Hmm, why ULL gets truncated to unsigned int here?
> > > No particular reason, but it should be better to not truncate here. Will
> > > fix it.
> > Yea, and looks like we are going to do with:
> > static inline u64 arm_smmu_strtab_num_sids(struct arm_smmu_device *smmu);
> >
> > Then let's be careful at those return-value holders too:
> > -----------------------------------------------------------
> > static int arm_smmu_init_strtab_linear(struct arm_smmu_device *smmu)
> > {
> > 	u32 size;
> > 	struct arm_smmu_strtab_cfg *cfg = &smmu->strtab_cfg;
> >
> > 	size = (1 << smmu->sid_bits) * sizeof(struct arm_smmu_ste);
> >          ^^^^
> >          overflow?
> > [...]
> > 	cfg->linear.num_ents = 1 << smmu->sid_bits;
> >                      ^^^^^^^^
> >                      This is u32
> > -----------------------------------------------------------
> It would make some sense to have something like:
>
>   u64 size = arm_smmu_strtab_max_sid()
>
>   /* Would require too much memory */
>   if (size > SZ_512M)
>      return -EINVAL;

Why not just check smmu->sid_bits?

For example,

if (smmu->sid_bits > 28)
    return -EINVAL;

The check can happen before the shift.

> Just to reject bad configuration rather than truncate the allocation
> and overflow STE array memory or something. Having drivers be robust
> to this kind of stuff is a confidential compute topic :\
>
> Jason