linux-kernel - Re: [PATCH] iommu/arm-smmu-v3: Fix last_sid_idx calculation for sid

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Message-ID: <CAE2F3rAQ8BpYcZZBS6BfFeZtMkH9LK7WZ7nniJSbTBW4xDq_rQ@mail.gmail.com>
Date: Wed, 2 Oct 2024 10:58:14 -0700
From: Daniel Mentz <danielmentz@...gle.com>
To: Nicolin Chen <nicolinc@...dia.com>, Yang Shi <yang@...amperecomputing.com>
Cc: iommu@...ts.linux.dev, linux-kernel@...r.kernel.org, 
	Jason Gunthorpe <jgg@...dia.com>, Ryan Huang <tzukui@...gle.com>, Will Deacon <will@...nel.org>, 
	Mostafa Saleh <smostafa@...gle.com>
Subject: Re: [PATCH] iommu/arm-smmu-v3: Fix last_sid_idx calculation for sid_bits==32

On Tue, Oct 1, 2024 at 9:05 PM Nicolin Chen <nicolinc@...dia.com> wrote:
>
> On Tue, Oct 01, 2024 at 06:53:57PM -0700, Daniel Mentz wrote:
> >
> > The function arm_smmu_init_strtab_2lvl uses the expression
> >
> > ((1 << smmu->sid_bits) - 1)
> >
> > to calculate the largest StreamID value. However, this fails for the
> > maximum allowed value of SMMU_IDR1.SIDSIZE which is 32. The C standard
> > states:
> >
> > "If the value of the right operand is negative or is greater than or
> > equal to the width of the promoted left operand, the behavior is
> > undefined."
> >
> > With smmu->sid_bits being 32, the prerequisites for undefined behavior
> > are met.  We observed that the value of (1 << 32) is 1 and not 0 as we
> > initially expected.
> >
> > Similar bit shift operations in arm_smmu_init_strtab_linear seem to not
> > be affected, because it appears to be unlikely for an SMMU to have
> > SMMU_IDR1.SIDSIZE set to 32 but then not support 2-level Stream tables
> >
> > This issue was found by Ryan Huang <tzukui@...gle.com> on our team.
>
> There is a patch that's sent a few hours earlier :)
> https://lore.kernel.org/linux-arm-kernel/20241001180346.1485194-1-yang@os.amperecomputing.com/

Thanks Nicolin.

Yang, in your change, I believe you are arguing based on the Arm spec
("dest = src << (shift % 32)"). Consider mentioning that the C
standard states that this behavior is undefined.

>
> Thanks
> Nicolin
>
> > Fixes: ce410410f1a7 ("iommu/arm-smmu-v3: Add arm_smmu_strtab_l1/2_idx()")
> > Signed-off-by: Daniel Mentz <danielmentz@...gle.com>
> > ---
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > index 737c5b882355..b55327d6058e 100644
> > --- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
> > @@ -3625,7 +3625,7 @@ static int arm_smmu_init_strtab_2lvl(struct arm_smmu_device *smmu)
> >         u32 l1size;
> >         struct arm_smmu_strtab_cfg *cfg = &smmu->strtab_cfg;
> >         unsigned int last_sid_idx =
> > -               arm_smmu_strtab_l1_idx((1 << smmu->sid_bits) - 1);
> > +               arm_smmu_strtab_l1_idx((1ULL << smmu->sid_bits) - 1);
> >
> >         /* Calculate the L1 size, capped to the SIDSIZE. */
> >         cfg->l2.num_l1_ents = min(last_sid_idx + 1, STRTAB_MAX_L1_ENTRIES);
> > --
> > 2.46.1.824.gd892dcdcdd-goog
> >