[PATCH] misc: issei: check bus message length before reading the command

[Linmao Li]

__issei_ham_process_ham_rsp() dispatches on hdr->cmd before the message
length is validated. The length comes from the firmware-owned DMA header
read in issei_dma_read(), which only bounds it from above, so firmware
sending a short bus message reaches the dispatch with less than
sizeof(struct ham_bus_message) bytes available.

For a zero-length message kmemdup() returns ZERO_SIZE_PTR, which passes
the NULL check in issei_dma_read(), and the dispatch dereferences it. A
length of one to three bytes gives a slab out-of-bounds read instead.

Reject bus messages shorter than the header before touching it, the way
the individual response handlers already validate their own length.

Fixes: 7bd4b9991db20 ("issei: implement main thread and ham messages")
Signed-off-by: Linmao Li <lilinmao@xxxxxxxxxx>
---
 drivers/misc/issei/ham.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/misc/issei/ham.c b/drivers/misc/issei/ham.c
index 17eae91f077d6..674d9733d16cf 100644
--- a/drivers/misc/issei/ham.c
+++ b/drivers/misc/issei/ham.c
@@ -132,6 +132,12 @@ static int __issei_ham_process_ham_rsp(struct issei_device *idev, const u8 *buf,
 {
 	struct ham_bus_message *hdr = (struct ham_bus_message *)buf;
 
+	if (length < sizeof(*hdr)) {
+		dev_err(&idev->dev, "Small bus message size %zu < %zu\n",
+			length, sizeof(*hdr));
+		return -EPROTO;
+	}
+
 	switch (hdr->cmd) {
 	case HAM_BUS_CMD_START_RSP:
 		return issei_ham_start_rsp(idev, buf, length);
-- 
2.25.1