Re: [PATCH] fbdev: udlfb: validate vendor descriptor items

[Helge Deller <deller@gmx.de>]

On 7/6/26 11:30, Pengpeng Hou wrote:
> dlfb_parse_vendor_descriptor() walks key-length-value items inside the
> DisplayLink vendor descriptor.
> 
> Require each item to contain its key, length and declared value bytes
> before reading item-specific fields such as max_area.
> 
> Signed-off-by: Pengpeng Hou <pengpeng@iscas.ac.cn>
> ---
>   drivers/video/fbdev/udlfb.c | 22 ++++++++++++++++------
>   1 file changed, 16 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/video/fbdev/udlfb.c b/drivers/video/fbdev/udlfb.c
> index fdbb8671a810..e78d6f95c9c5 100644
> --- a/drivers/video/fbdev/udlfb.c
> +++ b/drivers/video/fbdev/udlfb.c
> @@ -1586,19 +1586,29 @@ static int dlfb_parse_vendor_descriptor(struct dlfb_data *dlfb,
>   		desc += 5; /* the fixed header we've already parsed */
>   
>   		while (desc < desc_end) {
> +			char *value;
>   			u8 length;
>   			u16 key;
>   
> -			key = *desc++;
> -			key |= (u16)*desc++ << 8;
> +			if (desc_end - desc < sizeof(key) + sizeof(length))
> +				goto unrecognized;
> +
> +			key = get_unaligned_le16(desc);

Is there a reason why you switch to unconditional little-endian reads?
Is this "vendor descriptor" always little-endian?
If yes, then your patch is probably correct.
If not, I think your patch will most likely break big-endian machines.

Helge

> +			desc += sizeof(key);
>   			length = *desc++;
>   
> +			if (length > desc_end - desc)
> +				goto unrecognized;
> +
> +			value = desc;
>   			switch (key) {
>   			case 0x0200: { /* max_area */
> -				u32 max_area = *desc++;
> -				max_area |= (u32)*desc++ << 8;
> -				max_area |= (u32)*desc++ << 16;
> -				max_area |= (u32)*desc++ << 24;
> +				u32 max_area;
> +
> +				if (length < sizeof(max_area))
> +					goto unrecognized;
> +
> +				max_area = get_unaligned_le32(value);
>   				dev_warn(&intf->dev,
>   					 "DL chip limited to %d pixel modes\n",
>   					 max_area);