firmware: arm_scmi: Harden shared memory access in fetch_notification

[ Upstream commit 9bae076cd4e3e3c3dc185cae829d80b2dddec86e ]

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@arm.com>
Link: https://lore.kernel.org/r/20221222183823.518856-4-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/drivers/firmware/arm_scmi/shmem.c b/drivers/firmware/arm_scmi/shmem.c
index 415ef7d..56a1f61 100644 (file)
--- a/drivers/firmware/arm_scmi/shmem.c
+++ b/drivers/firmware/arm_scmi/shmem.c
@@ -71,8 +71,10 @@ 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);