linux-kernel - [PATCH 2/2] drm/bridge: ti-sn65dsi86: Never increase the length when reading from AUX

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Date:   Mon, 11 Dec 2023 16:55:27 -0800
From:   Douglas Anderson <dianders@...omium.org>
To:     dri-devel@...ts.freedesktop.org
Cc:     Douglas Anderson <dianders@...omium.org>,
        Andrzej Hajda <andrzej.hajda@...el.com>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...il.com>,
        Jernej Skrabec <jernej.skrabec@...il.com>,
        Jonas Karlman <jonas@...boo.se>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Neil Armstrong <neil.armstrong@...aro.org>,
        Robert Foss <rfoss@...nel.org>,
        Sam Ravnborg <sam@...nborg.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] drm/bridge: ti-sn65dsi86: Never increase the length when reading from AUX

For aux reads, the value `msg->size` indicates the size of the buffer
provided by `msg->buffer`. We should never in any circumstances write
more bytes to the buffer since it may overflow the buffer.

In the ti-sn65dsi86 driver there is one code path that reads the
transfer length from hardware. Even though it's never been seen to be
a problem, we should make extra sure that the hardware isn't
increasing the length since doing so would cause us to overrun the
buffer.

Fixes: 982f589bde7a ("drm/bridge: ti-sn65dsi86: Update reply on aux failures")
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---

 drivers/gpu/drm/bridge/ti-sn65dsi86.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/ti-sn65dsi86.c b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
index 5b8e1dfc458d..7ff465241267 100644
--- a/drivers/gpu/drm/bridge/ti-sn65dsi86.c
+++ b/drivers/gpu/drm/bridge/ti-sn65dsi86.c
@@ -527,6 +527,7 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
 	u32 request_val = AUX_CMD_REQ(msg->request);
 	u8 *buf = msg->buffer;
 	unsigned int len = msg->size;
+	unsigned int short_len;
 	unsigned int val;
 	int ret;
 	u8 addr_len[SN_AUX_LENGTH_REG + 1 - SN_AUX_ADDR_19_16_REG];
@@ -600,7 +601,8 @@ static ssize_t ti_sn_aux_transfer(struct drm_dp_aux *aux,
 	}
 
 	if (val & AUX_IRQ_STATUS_AUX_SHORT) {
-		ret = regmap_read(pdata->regmap, SN_AUX_LENGTH_REG, &len);
+		ret = regmap_read(pdata->regmap, SN_AUX_LENGTH_REG, &short_len);
+		len = min(len, short_len);
 		if (ret)
 			goto exit;
 	} else if (val & AUX_IRQ_STATUS_NAT_I2C_FAIL) {
-- 
2.43.0.472.g3155946c3a-goog