Re: [PATCH v4 2/6] iommu/arm-smmu-v3-iommufd: Reject unsupported bits in invalidation commands

[Jason Gunthorpe]

On Tue, Jul 14, 2026 at 11:48:48AM -0700, Nicolin Chen wrote:
> 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: a reserved or
> undefined bit makes a 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 stops the host from forwarding any bit whose meaning it does
> not control. Document this contract in the uAPI header, so user space must
> take the responsibility to forward valid commands only.
> 
> Some fields and whole opcodes are legal only on an SMMU that implements
> the matching feature, so accept them conditionally:
> 
>  - NUM, SCALE, TG and TTL need FEAT_RANGE_INV.
>  - SCALE bit 25, for values above 31, and TTL == 0b01 with a 16KB TG
>    need SMMU_IDR5.DS, gated on ARM_SMMU_FEAT_DS so that a VMM exposing
>    DS from the reported IDR5 register keeps working.
>  - ASID is limited to asid_bits, since its upper 8 bits are RES0 on an
>    SMMU that only supports 8-bit ASIDs.
>  - ATC_INV needs FEAT_ATS. Per the 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.
> 
>  - SSV, SSID and Global need a non-zero ssid_bits. Without it, setting
>    them is not illegal but CONSTRAINED UNPREDICTABLE, which a guest
>    should not be able to provoke. Global also takes effect only when
>    SSV == 1, broadening the invalidation from the one SSID to all the
>    PASIDs of the single device that the SID field addresses.
> 
> Some values inside the accepted fields are Reserved too:
> 
>  - NUM == 0, SCALE == 0 and TTL == 0 together are a Reserved combination
>    and cause a CERROR_ILL.
>  - NUM, SCALE and TTL turn RES0 when TG == 0.
>  - An ATC_INV Size above 52, the invalidate-all span, is permitted to
>    raise a CERROR_ILL.
> 
> Reject these Reserved values the same way. In contrast, an out-of-range
> address or ID value is defined as CONSTRAINED UNPREDICTABLE that would
> be scoped to the guest itself, so it does not deserve a check.
> 
> Fixes: d68beb276ba2 ("iommu/arm-smmu-v3: Support IOMMU_HWPT_INVALIDATE using a VIOMMU object")
> Cc: stable@xxxxxxxxxxxxxxx
> Assisted-by: Claude:claude-fable-5
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> ---
>  include/uapi/linux/iommufd.h                  |   4 +-
>  .../arm/arm-smmu-v3/arm-smmu-v3-iommufd.c     | 125 ++++++++++++++++--
>  2 files changed, 117 insertions(+), 12 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@xxxxxxxxxx>

Jason