firmware: arm_scmi: Harden shared memory access in fetch_response

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

Harden shmem_fetch_response to properly truncate such a bad messages.

Fixes: 5c8a47a5a91d ("firmware: arm_scmi: Make scmi core independent of the transport type")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222183823.518856-3-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>

diff --git a/drivers/firmware/arm_scmi/shmem.c b/drivers/firmware/arm_scmi/shmem.c
index 1dfe534..135f871 100644 (file)
--- a/drivers/firmware/arm_scmi/shmem.c
+++ b/drivers/firmware/arm_scmi/shmem.c
@@ -81,10 +81,11 @@ u32 shmem_read_header(struct scmi_shared_mem __iomem *shmem)
 void shmem_fetch_response(struct scmi_shared_mem __iomem *shmem,
                          struct scmi_xfer *xfer)
 {
+       size_t len = ioread32(&shmem->length);
+
        xfer->hdr.status = ioread32(shmem->msg_payload);
        /* Skip the length of header and status in shmem area i.e 8 bytes */
-       xfer->rx.len = min_t(size_t, xfer->rx.len,
-                            ioread32(&shmem->length) - 8);
+       xfer->rx.len = min_t(size_t, xfer->rx.len, len > 8 ? len - 8 : 0);
 
        /* Take a copy to the rx buffer.. */
        memcpy_fromio(xfer->rx.buf, shmem->msg_payload + 4, xfer->rx.len);