[PATCH v2 1/2] staging: nvec: fix use-after-free in nvec_rx_completed()

[Alexandru Hossu <hossu.alexandru@gmail.com>]

In nvec_rx_completed(), when an incomplete RX transfer is detected,
nvec_msg_free() is called to return the message back to the pool by
clearing its 'used' atomic flag. Immediately after this, the code
accesses nvec->rx->data[0] to check the message type.

Since nvec_msg_free() marks the pool slot as available via atomic_set(),
any concurrent or subsequent call to nvec_msg_alloc() could claim that
same slot and overwrite its data[] array. Reading nvec->rx->data[0] after
freeing the message is therefore a use-after-free.

Fix this by saving the message type byte before calling nvec_msg_free(),
then using the saved value for the battery quirk check.

Fixes: d6bdcf2e1019 ("staging: nvec: Add battery quirk to ignore incomplete responses")
Reviewed-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Alexandru Hossu <hossu.alexandru@gmail.com>
---
v2:
 - Add Fixes tag (Dan Carpenter)
 - Remove obvious comment (Dan Carpenter)
 - Move declaration to start of block (Dan Carpenter)

 drivers/staging/nvec/nvec.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/staging/nvec/nvec.c b/drivers/staging/nvec/nvec.c
index 952c5a849a56..2a3499dd4d63 100644
--- a/drivers/staging/nvec/nvec.c
+++ b/drivers/staging/nvec/nvec.c
@@ -494,6 +494,8 @@ static void nvec_tx_completed(struct nvec_chip *nvec)
 static void nvec_rx_completed(struct nvec_chip *nvec)
 {
 	if (nvec->rx->pos != nvec_msg_size(nvec->rx)) {
+		unsigned char msg_type = nvec->rx->data[0];
+
 		dev_err(nvec->dev, "RX incomplete: Expected %u bytes, got %u\n",
 			(uint)nvec_msg_size(nvec->rx),
 			(uint)nvec->rx->pos);
@@ -502,7 +504,7 @@ static void nvec_rx_completed(struct nvec_chip *nvec)
 		nvec->state = 0;
 
 		/* Battery quirk - Often incomplete, and likes to crash */
-		if (nvec->rx->data[0] == NVEC_BAT)
+		if (msg_type == NVEC_BAT)
 			complete(&nvec->ec_transfer);
 
 		return;
-- 
2.53.0