Re: [PATCH 1/6] mailbox: pcc: Validate shared memory signature on request

["lihuisong (C)" <lihuisong@huawei.com>]

On 6/28/2026 12:37 AM, Sudeep Holla wrote:
> ACPI defines the PCC shared memory signature as the bitwise OR of
> 0x50434300 and the PCC subspace ID. The signature is located at byte
> offset 0 for the generic, extended and reduced PCC shared memory region
> layouts.
>
> Validate the signature when a client requests a PCC mailbox channel, after
> mapping the shared memory and before binding the mailbox client. This
> keeps PCC shared memory validation in the PCC mailbox controller instead
> of duplicating it in individual clients.
>
> ACPI 6.6 added clarification that the signature is populated by the
> platform and verified by OSPM.
>
> Cc: Jassi Brar <jassisinghbrar@gmail.com>
> Cc: Huisong Li <lihuisong@huawei.com>
> Signed-off-by: Sudeep Holla <sudeep.holla@kernel.org>
> ---
>   drivers/mailbox/pcc.c | 38 +++++++++++++++++++++++++++++++++-----
>   1 file changed, 33 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/mailbox/pcc.c b/drivers/mailbox/pcc.c
> index 636879ae1db7..6fbc17d41774 100644
> --- a/drivers/mailbox/pcc.c
> +++ b/drivers/mailbox/pcc.c
> @@ -345,6 +345,28 @@ static irqreturn_t pcc_mbox_irq(int irq, void *p)
>   	return IRQ_HANDLED;
>   }
>   
> +static int pcc_mbox_validate_signature(struct pcc_mbox_chan *pcc_mchan,
> +				       int subspace_id)
> +{
> +	u32 expected_signature = PCC_SIGNATURE | subspace_id;
> +	u32 signature;
> +
> +	if (pcc_mchan->shmem_size < sizeof(signature)) {
> +		pr_err("PCC subspace %d shared memory is too small\n",
> +		       subspace_id);
> +		return -EINVAL;
> +	}
> +
> +	signature = ioread32(pcc_mchan->shmem);
> +	if (signature != expected_signature) {
> +		pr_err("PCC subspace %d has invalid signature: %#x expected %#x\n",
> +		       subspace_id, signature, expected_signature);
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
If the signature is incorrect, there must be an issue with the PCCT 
table reporting.
So how about place this verification in pcc_mbox_probe?
We can mark the channel as unavailable so that other normal channels can 
continue to be used.
And the impact is minimal.
>   /**
>    * pcc_mbox_request_channel - PCC clients call this function to
>    *		request a pointer to their PCC subspace, from which they
> @@ -381,14 +403,20 @@ pcc_mbox_request_channel(struct mbox_client *cl, int subspace_id)
>   	if (!pcc_mchan->shmem)
>   		return ERR_PTR(-ENXIO);
>   
> +	rc = pcc_mbox_validate_signature(pcc_mchan, subspace_id);
> +	if (rc)
> +		goto err_unmap_shmem;
> +
>   	rc = mbox_bind_client(chan, cl);
> -	if (rc) {
> -		iounmap(pcc_mchan->shmem);
> -		pcc_mchan->shmem = NULL;
> -		return ERR_PTR(rc);
> -	}
> +	if (rc)
> +		goto err_unmap_shmem;
>   
>   	return pcc_mchan;
> +
> +err_unmap_shmem:
> +	iounmap(pcc_mchan->shmem);
> +	pcc_mchan->shmem = NULL;
> +	return ERR_PTR(rc);
>   }
>   EXPORT_SYMBOL_GPL(pcc_mbox_request_channel);
>   
>