[PATCH 3/5] firmware: arm_scmi: Harden shared memory access in fetch_notification

[Cristian Marussi]

A misbheaving SCMI platform firmware could reply with out-of-spec
notifications, shorter than the mimimum size comprising a header.

Fixes: d5141f37c42e ("firmware: arm_scmi: Add notifications support in transport layer")
Signed-off-by: Cristian Marussi <cristian.marussi at arm.com>
---
 drivers/firmware/arm_scmi/shmem.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/firmware/arm_scmi/shmem.c b/drivers/firmware/arm_scmi/shmem.c
index 135f8718000f..87b4f4d35f06 100644
--- a/drivers/firmware/arm_scmi/shmem.c
+++ b/drivers/firmware/arm_scmi/shmem.c
@@ -94,8 +94,10 @@ void shmem_fetch_response(struct scmi_shared_mem __iomem *shmem,
 void shmem_fetch_notification(struct scmi_shared_mem __iomem *shmem,
 			      size_t max_len, struct scmi_xfer *xfer)
 {
+	size_t len = ioread32(&shmem->length);
+
 	/* Skip only the length of header in shmem area i.e 4 bytes */
-	xfer->rx.len = min_t(size_t, max_len, ioread32(&shmem->length) - 4);
+	xfer->rx.len = min_t(size_t, max_len, len > 4 ? len - 4 : 0);
 
 	/* Take a copy to the rx buffer.. */
 	memcpy_fromio(xfer->rx.buf, shmem->msg_payload, xfer->rx.len);
-- 
2.34.1