RE: [PATCH v1 1/5] iommu/arm-smmu-v3-iommufd: Reject unsupported bits in invalidation commands

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Nicolin Chen <nicolinc@nvidia.com>
> Sent: Tuesday, June 30, 2026 5:16 AM
> 
> The arm_vsmmu_cache_invalidate() op hands a guest's invalidation
> commands
> to the trusted main command queue after enforcing only the VMID or the
> SID,
> and passes the rest of the command through to the queue unchanged.
> 
> That lets a guest set bits the host never meant to forward, in two ways. A
> bit can take the command out of the guest's own scope: the ATC_INV Global
> bit, for one, makes the SMMU ignore the SID and invalidate the ATC of every
> device, not just the guest's. A reserved or undefined bit instead makes the
> command malformed; per the Arm SMMUv3 specification, in its section 4.1.3
> "Command errors", a CERROR_ILL is raised, among other cases, when:
> 
>   A valid command opcode is used and a Reserved or undefined field is
>   optionally detected as non-zero, which results in the command being
>   treated as malformed.
> 
> Restrict each opcode to the fields that the driver supports and reject the
> command with -EIO if it sets any other bit, before the command reaches the
> queue. This keeps a guest scoped to its own devices and stops the host from
> forwarding any bit whose meaning it does not control.
> 
> Some fields and whole opcodes are legal only on an SMMU that implements
> the
> matching feature, so accept them conditionally. The NUM, SCALE and TG
> range
> fields need FEAT_RANGE_INV. The ATC_INV opcode needs FEAT_ATS. Per the
> same
> specification's section 4.5 "ATS and PRI", CMD_ATC_INV is ILLEGAL when:
> 
>   SMMU_IDR0.ATS == 0 and this command is issued on a Non-secure or
> Secure
>   Command queue.
> 
> The SSV and SSID substream fields require a non-zero ssid_bits, so without
> substream support setting them is not illegal but CONSTRAINED
> UNPREDICTABLE,
> which a guest should not be able to provoke.
> 
> Fixes: d68beb276ba2 ("iommu/arm-smmu-v3: Support
> IOMMU_HWPT_INVALIDATE using a VIOMMU object")
> Cc: stable@vger.kernel.org
> Assisted-by: Claude:claude-opus-4-8
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>

Reviewed-by: Kevin Tian <kevin.tian@intel.com>

> +	case CMDQ_OP_ATC_INV:
> +		/*
> +		 * Exclude the Global bit: it makes the SMMU ignore the SID
> and
> +		 * invalidate the ATC of every device, not just the guest's.
> +		 */

I suppose the Global bit in vcmdq is guaranteed by HW to be contained within
the vcmdq, then the SMMU will convert it into multiple INV cmds sent to
devices associated with the vcmdq?

> +		allowed[0] |= CMDQ_ATC_0_SID;
> +		allowed[1] |= CMDQ_ATC_1_SIZE |
> CMDQ_ATC_1_ADDR_MASK;
> +		/* SSV/SSID require substream support */
> +		if (vsmmu->smmu->ssid_bits)
> +			allowed[0] |= CMDQ_0_SSV | CMDQ_ATC_0_SSID;
> +		break;

the latter check of ARM_SMMU_FEAT_ATS could be moved here before
setting any allowed bits for ATC_INV.