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

[Yang Shi <yang@os.amperecomputing.com>]

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