Re: [PATCH] iommu/arm-smmu-qcom: Fix mask extraction for bootloader programmed SMRs

[Robin Murphy]

On 2021-01-25 21:52, Isaac J. Manjarres wrote:
> When extracting the mask for a SMR that was programmed by the
> bootloader, the SMR's valid bit is also extracted and is treated
> as part of the mask, which is not correct. Consider the scenario
> where an SMMU master whose context is determined by a bootloader
> programmed SMR is removed (omitting parts of device/driver core):
>
> ->iommu_release_device()
>   -> arm_smmu_release_device()
>    -> arm_smmu_master_free_smes()
>     -> arm_smmu_free_sme() /* Assume that the SME is now free */
>     -> arm_smmu_write_sme()
>      -> arm_smmu_write_smr() /* Construct SMR value using mask and SID */
>
> Since the valid bit was considered as part of the mask, the SMR will
> be programmed as valid.

Ah, right, because ARM_SMMU_SMR_{ID,MASK} are 16-bit fields to 
accommodate EXIDS, which doesn't matter normally when the IDs are 
strictly validated in arm_smmu_probe_device()...

> Fix the SMR mask extraction step for bootloader programmed SMRs
> by masking out the valid bit when we know that we're already
> working with a valid SMR.

This seems like the neatest approach to me.

Reviewed-by: Robin Murphy <robin.murphy@xxxxxxx>

> Fixes: 07a7f2caaa5a ("iommu/arm-smmu-qcom: Read back stream mappings")
> Signed-off-by: Isaac J. Manjarres <isaacm@xxxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> ---
>   drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> index bcda170..abb1d2f 100644
> --- a/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> +++ b/drivers/iommu/arm/arm-smmu/arm-smmu-qcom.c
> @@ -206,6 +206,8 @@ static int qcom_smmu_cfg_probe(struct arm_smmu_device *smmu)
>   		smr = arm_smmu_gr0_read(smmu, ARM_SMMU_GR0_SMR(i));
>   
>   		if (FIELD_GET(ARM_SMMU_SMR_VALID, smr)) {
> +			/* Ignore valid bit for SMR mask extraction. */
> +			smr &= ~ARM_SMMU_SMR_VALID;
>   			smmu->smrs[i].id = FIELD_GET(ARM_SMMU_SMR_ID, smr);
>   			smmu->smrs[i].mask = FIELD_GET(ARM_SMMU_SMR_MASK, smr);
>   			smmu->smrs[i].valid = true;