[PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

[PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

[Douglas Anderson]

While testing, I happened to notice a random crash that looked like:

  Kernel panic - not syncing: stack-protector:
  Kernel stack is corrupted in: drm_dp_dpcd_probe+0x120/0x120

Analysis of drm_dp_dpcd_probe() shows that we pass in a 1-byte buffer
(allocated on the stack) to the aux->transfer() function. Presumably
if the aux->transfer() writes more than one byte to this buffer then
we're in a bad shape.

Dropping into kgdb, I noticed that "aux->transfer" pointed at
ps8640_aux_transfer().

Reading through ps8640_aux_transfer(), I can see that there are cases
where it could write more bytes to msg->buffer than were specified by
msg->size. This could happen if the hardware reported back something
bogus to us. Let's fix this so we never write more than msg->size
bytes. We'll still read all the bytes from the hardware just in case
the hardware requires it since the aux transfer data comes through an
auto-incrementing register.

NOTE: I have no actual way to reproduce this issue but it seems likely
this is what was happening in the crash I looked at.

Fixes: 13afcdd7277e ("drm/bridge: parade-ps8640: Add support for AUX channel")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- Still read all the bytes; just don't write them all to the buffer.

 drivers/gpu/drm/bridge/parade-ps8640.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
index 8161b1a1a4b1..dfb97fbb5be6 100644
--- a/drivers/gpu/drm/bridge/parade-ps8640.c
+++ b/drivers/gpu/drm/bridge/parade-ps8640.c
@@ -330,7 +330,8 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
 				return ret;
 			}
 
-			buf[i] = data;
+			if (i < msg->size)
+				buf[i] = data;
 		}
 	}
 
-- 
2.43.0.472.g3155946c3a-goog

[PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

[Douglas Anderson]

While testing, I happened to notice a random crash that looked like:

  Kernel panic - not syncing: stack-protector:
  Kernel stack is corrupted in: drm_dp_dpcd_probe+0x120/0x120

Analysis of drm_dp_dpcd_probe() shows that we pass in a 1-byte buffer
(allocated on the stack) to the aux->transfer() function. Presumably
if the aux->transfer() writes more than one byte to this buffer then
we're in a bad shape.

Dropping into kgdb, I noticed that "aux->transfer" pointed at
ps8640_aux_transfer().

Reading through ps8640_aux_transfer(), I can see that there are cases
where it could write more bytes to msg->buffer than were specified by
msg->size. This could happen if the hardware reported back something
bogus to us. Let's fix this so we never write more than msg->size
bytes. We'll still read all the bytes from the hardware just in case
the hardware requires it since the aux transfer data comes through an
auto-incrementing register.

NOTE: I have no actual way to reproduce this issue but it seems likely
this is what was happening in the crash I looked at.

Fixes: 13afcdd7277e ("drm/bridge: parade-ps8640: Add support for AUX channel")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
---

Changes in v2:
- Still read all the bytes; just don't write them all to the buffer.

 drivers/gpu/drm/bridge/parade-ps8640.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
index 8161b1a1a4b1..dfb97fbb5be6 100644
--- a/drivers/gpu/drm/bridge/parade-ps8640.c
+++ b/drivers/gpu/drm/bridge/parade-ps8640.c
@@ -330,7 +330,8 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
 				return ret;
 			}
 
-			buf[i] = data;
+			if (i < msg->size)
+				buf[i] = data;
 		}
 	}
 
-- 
2.43.0.472.g3155946c3a-goog

[PATCH v2 2/2] drm/bridge: ti-sn65dsi86: Never store more than msg->size bytes in AUX xfer

[Douglas Anderson]

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@chromium.org>
---

Changes in v2:
- Updated patch subject to match ps8640 patch.

 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 9095d1453710..62cc3893dca5 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

[PATCH v2 2/2] drm/bridge: ti-sn65dsi86: Never store more than msg->size bytes in AUX xfer

[Douglas Anderson]

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@chromium.org>
---

Changes in v2:
- Updated patch subject to match ps8640 patch.

 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 9095d1453710..62cc3893dca5 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

Re: [PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

[Doug Anderson]

Hi,

On Thu, Dec 14, 2023 at 12:32 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> While testing, I happened to notice a random crash that looked like:
>
>   Kernel panic - not syncing: stack-protector:
>   Kernel stack is corrupted in: drm_dp_dpcd_probe+0x120/0x120
>
> Analysis of drm_dp_dpcd_probe() shows that we pass in a 1-byte buffer
> (allocated on the stack) to the aux->transfer() function. Presumably
> if the aux->transfer() writes more than one byte to this buffer then
> we're in a bad shape.
>
> Dropping into kgdb, I noticed that "aux->transfer" pointed at
> ps8640_aux_transfer().
>
> Reading through ps8640_aux_transfer(), I can see that there are cases
> where it could write more bytes to msg->buffer than were specified by
> msg->size. This could happen if the hardware reported back something
> bogus to us. Let's fix this so we never write more than msg->size
> bytes. We'll still read all the bytes from the hardware just in case
> the hardware requires it since the aux transfer data comes through an
> auto-incrementing register.
>
> NOTE: I have no actual way to reproduce this issue but it seems likely
> this is what was happening in the crash I looked at.
>
> Fixes: 13afcdd7277e ("drm/bridge: parade-ps8640: Add support for AUX channel")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> Changes in v2:
> - Still read all the bytes; just don't write them all to the buffer.
>
>  drivers/gpu/drm/bridge/parade-ps8640.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> index 8161b1a1a4b1..dfb97fbb5be6 100644
> --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> @@ -330,7 +330,8 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
>                                 return ret;
>                         }
>
> -                       buf[i] = data;
> +                       if (i < msg->size)
> +                               buf[i] = data;

Ah crud. I just read over Stephen's response again and realized that
in this case I'm returning the wrong length from the function. A v3
will come shortly.

-Doug

Re: [PATCH v2 1/2] drm/bridge: parade-ps8640: Never store more than msg->size bytes in AUX xfer

[Doug Anderson]

Hi,

On Thu, Dec 14, 2023 at 12:32 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> While testing, I happened to notice a random crash that looked like:
>
>   Kernel panic - not syncing: stack-protector:
>   Kernel stack is corrupted in: drm_dp_dpcd_probe+0x120/0x120
>
> Analysis of drm_dp_dpcd_probe() shows that we pass in a 1-byte buffer
> (allocated on the stack) to the aux->transfer() function. Presumably
> if the aux->transfer() writes more than one byte to this buffer then
> we're in a bad shape.
>
> Dropping into kgdb, I noticed that "aux->transfer" pointed at
> ps8640_aux_transfer().
>
> Reading through ps8640_aux_transfer(), I can see that there are cases
> where it could write more bytes to msg->buffer than were specified by
> msg->size. This could happen if the hardware reported back something
> bogus to us. Let's fix this so we never write more than msg->size
> bytes. We'll still read all the bytes from the hardware just in case
> the hardware requires it since the aux transfer data comes through an
> auto-incrementing register.
>
> NOTE: I have no actual way to reproduce this issue but it seems likely
> this is what was happening in the crash I looked at.
>
> Fixes: 13afcdd7277e ("drm/bridge: parade-ps8640: Add support for AUX channel")
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---
>
> Changes in v2:
> - Still read all the bytes; just don't write them all to the buffer.
>
>  drivers/gpu/drm/bridge/parade-ps8640.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> index 8161b1a1a4b1..dfb97fbb5be6 100644
> --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> +++ b/drivers/gpu/drm/bridge/parade-ps8640.c
> @@ -330,7 +330,8 @@ static ssize_t ps8640_aux_transfer_msg(struct drm_dp_aux *aux,
>                                 return ret;
>                         }
>
> -                       buf[i] = data;
> +                       if (i < msg->size)
> +                               buf[i] = data;

Ah crud. I just read over Stephen's response again and realized that
in this case I'm returning the wrong length from the function. A v3
will come shortly.

-Doug

Re: [PATCH v2 2/2] drm/bridge: ti-sn65dsi86: Never store more than msg->size bytes in AUX xfer

[Robert Foss]

On Thu, Dec 14, 2023 at 9:32 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> 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@chromium.org>
> ---
>
> Changes in v2:
> - Updated patch subject to match ps8640 patch.
>
>  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 9095d1453710..62cc3893dca5 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
>
>

Reviewed-by: Robert Foss <rfoss@kernel.org>

Re: [PATCH v2 2/2] drm/bridge: ti-sn65dsi86: Never store more than msg->size bytes in AUX xfer

[Robert Foss]

On Thu, Dec 14, 2023 at 9:32 PM Douglas Anderson <dianders@chromium.org> wrote:
>
> 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@chromium.org>
> ---
>
> Changes in v2:
> - Updated patch subject to match ps8640 patch.
>
>  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 9095d1453710..62cc3893dca5 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
>
>

Reviewed-by: Robert Foss <rfoss@kernel.org>