Subject: Re: [PATCH 3/9] iommu/arm-smmu-v3: Use the HW arm_smmu_cmd in cmdq submission functions

From: Jason Gunthorpe <[email-masked]>
Date: Fri, 8 May 2026 13:00:41 -0300

On Fri, May 08, 2026 at 08:27:26AM +0000, Pranjal Shrivastava wrote:
> >  /* Should be installed after arm_smmu_install_ste_for_dev() */
> > @@ -4823,7 +4826,8 @@ static int arm_smmu_device_reset(struct arm_smmu_device *smmu)
> >  {
> >  	int ret;
> >  	u32 reg, enables;
> > -	struct arm_smmu_cmdq_ent cmd;
> > +	struct arm_smmu_cmdq_ent ent;
> 
> This shouldn't be uninitialized, we only seem to be setting ent.opcode
> later in the function. 

Yes, that's how the existing code is.

        struct arm_smmu_cmdq_ent cmd;

        cmd.opcode = CMDQ_OP_CFGI_ALL;
        arm_smmu_cmdq_issue_cmd_with_sync(smmu, &cmd);

                cmd.opcode = CMDQ_OP_TLBI_EL2_ALL;
                arm_smmu_cmdq_issue_cmd_with_sync(smmu, &cmd);

        cmd.opcode = CMDQ_OP_TLBI_NSNH_ALL;
        arm_smmu_cmdq_issue_cmd_with_sync(smmu, &cmd);

> Since arm_smmu_cmdq_build_cmd reads other fields
> of ent to build the cmd, we are potentially sending stack garbage in ent

Ah, it is tricky, it doesn't:

static int arm_smmu_cmdq_build_cmd(u64 *cmd, struct arm_smmu_cmdq_ent *ent)
{
        memset(cmd, 0, 1 << CMDQ_ENT_SZ_SHIFT);
        cmd[0] |= FIELD_PREP(CMDQ_0_OP, ent->opcode);

        switch (ent->opcode) {
        case CMDQ_OP_TLBI_EL2_ALL:
        case CMDQ_OP_TLBI_NSNH_ALL:
                break;
[..]
        case CMDQ_OP_CFGI_ALL:
                /* Cover the entire SID range */
                cmd[1] |= FIELD_PREP(CMDQ_CFGI_1_RANGE, 31);
                break;

Only opcode is used, so it's "fine"

Later patches remove ent and this trickyness so let's just leave it:

	/* Invalidate any cached configuration */
	arm_smmu_cmdq_issue_cmd_with_sync(smmu, arm_smmu_make_cmd_cfgi_all());

	/* Invalidate any stale TLB entries */
	if (smmu->features & ARM_SMMU_FEAT_HYP) {
		arm_smmu_cmdq_issue_cmd_with_sync(
			smmu, arm_smmu_make_cmd_op(CMDQ_OP_TLBI_EL2_ALL));
	}

	arm_smmu_cmdq_issue_cmd_with_sync(
		smmu, arm_smmu_make_cmd_op(CMDQ_OP_TLBI_NSNH_ALL));

Jason