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

[Sudeep Holla]

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@xxxxxxxxx>
Cc: Huisong Li <lihuisong@xxxxxxxxxx>
Signed-off-by: Sudeep Holla <sudeep.holla@xxxxxxxxxx>
---
 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;
+}
+
 /**
  * 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);
 

-- 
2.43.0