[PATCH v2 5/6] firmware: samsung: acpm: Fix out-of-bounds read and infinite loop in RX path

[Tudor Ambarus]

Sashiko identified these bugs in [1].

The ACPM driver reads the rx_front and rx_rear pointers directly from
SRAM and uses them to calculate SRAM offsets and loop termination
conditions.

If a firmware bug writes a value greater than or equal to the queue
length (achan->qlen) at those addresses, two failures occur:

1. Out-of-bounds read: The rear pointer ('i') is used to calculate the
   MMIO address before the modulo operation is applied, leading to an
   immediate out-of-bounds memory access.
2. Infinite loop: The loop iterates using 'i = (i + 1) % achan->qlen'.
   Because 'i' is mathematically capped below qlen, if 'rx_front' is
   greater than or equal to qlen, 'i' will never equal 'rx_front'.
   The CPU will spin forever, holding the rx_lock and deadlocking the
   polling thread.

Protect the kernel by strictly validating the MMIO queue offsets
immediately after reading them.

Cc: stable at vger.kernel.org
Fixes: a88927b534ba ("firmware: add Exynos ACPM protocol driver")
Closes: https://sashiko.dev/#/patchset/20260420-acpm-tmu-v3-0-3dc8e93f0b26%40linaro.org [1]
Signed-off-by: Tudor Ambarus <tudor.ambarus at linaro.org>
---
 drivers/firmware/samsung/exynos-acpm.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/firmware/samsung/exynos-acpm.c b/drivers/firmware/samsung/exynos-acpm.c
index c9aa79c2faa4..43658cc1347a 100644
--- a/drivers/firmware/samsung/exynos-acpm.c
+++ b/drivers/firmware/samsung/exynos-acpm.c
@@ -230,6 +230,13 @@ static int acpm_get_rx(struct acpm_chan *achan, const struct acpm_xfer *xfer)
 	rx_front = readl(achan->rx.front);
 	i = readl(achan->rx.rear);
 
+	if (rx_front >= achan->qlen || i >= achan->qlen) {
+		dev_err(achan->acpm->dev,
+			"Invalid RX queue pointers from firmware: front=%u rear=%u qlen=%u\n",
+			rx_front, i, achan->qlen);
+		return -EIO;
+	}
+
 	tx_seqnum = FIELD_GET(ACPM_PROTOCOL_SEQNUM, xfer->txd[0]);
 
 	if (i == rx_front) {

-- 
2.54.0.rc2.544.gc7ae2d5bb8-goog