Re: [PATCH] drm/dp/mst: fix OOB reads in remote DPCD/I2C sideband reply parsers

[lyude]

Hi - one comment down below:

On Fri, 2026-04-10 at 03:41 +0000, Ashutosh Desai wrote:
> drm_dp_sideband_parse_remote_dpcd_read() reads num_bytes from the raw
> message and then unconditionally does:
> 
>   memcpy(bytes, &raw->msg[idx], num_bytes);
> 
> without checking that idx + num_bytes <= raw->curlen.  raw->msg[] is
> 256 bytes; if a malicious or misbehaving MST hub sets num_bytes
> larger
> than the remaining payload, the memcpy reads past the received data
> into whatever follows in raw->msg[].
> 
> drm_dp_sideband_parse_remote_i2c_read_ack() has the same flaw and
> also
> lacked the post-increment idx bounds check (noted with a /* TODO
> check */
> comment since the code was introduced).
> 
> Add the missing bounds checks in both functions, consistent with the
> pattern used throughout the rest of the sideband parser.
> 
> Signed-off-by: Ashutosh Desai <ashutoshdesai993@xxxxxxxxx>
> ---
>  drivers/gpu/drm/display/drm_dp_mst_topology.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> index 170113520..9cef6c576 100644
> --- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
> +++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
> @@ -873,6 +873,8 @@ static bool
> drm_dp_sideband_parse_remote_dpcd_read(struct drm_dp_sideband_msg_rx
>  	idx++;
>  	if (idx > raw->curlen)
>  		goto fail_len;
> +	if (idx + repmsg->u.remote_dpcd_read_ack.num_bytes > raw-
> >curlen)
> +		goto fail_len;

Do we need both checks here? It seems like we could just replace the
first check with the second check.

>  
>  	memcpy(repmsg->u.remote_dpcd_read_ack.bytes, &raw->msg[idx],
> repmsg->u.remote_dpcd_read_ack.num_bytes);
>  	return true;
> @@ -907,7 +909,11 @@ static bool
> drm_dp_sideband_parse_remote_i2c_read_ack(struct drm_dp_sideband_msg
>  		goto fail_len;
>  	repmsg->u.remote_i2c_read_ack.num_bytes = raw->msg[idx];
>  	idx++;
> -	/* TODO check */
> +	if (idx > raw->curlen)
> +		goto fail_len;
> +	if (idx + repmsg->u.remote_i2c_read_ack.num_bytes > raw-
> >curlen)
> +		goto fail_len;

Same here

> +
>  	memcpy(repmsg->u.remote_i2c_read_ack.bytes, &raw->msg[idx],
> repmsg->u.remote_i2c_read_ack.num_bytes);
>  	return true;
>  fail_len: