Re: [RFC PATCH] media: uvcvideo: compute frame buffer size in 64-bit arithmetic

[Ricardo Ribalda]

Hi Natasha

This has been previously discussed in the list:
https://lore.kernel.org/linux-media/20260812103251.18309-1-noambs2999@xxxxxxxxx/

Maybe you can help reviewing it?

Thanks!

On Mon, 17 Aug 2026 at 14:40, Natasha Klaus
<natalie.klaus@xxxxxxxxxxxxxxxxxxxxxxx> wrote:
>
> uvc_parse_frame() computes the default frame buffer size as
>
>         frame->dwMaxVideoFrameBufferSize = format->bpp * frame->wWidth
>                                          * frame->wHeight / 8;
>
> format->bpp is u8 and wWidth and wHeight are u16, so all three operands
> promote to signed int and the product is evaluated in 32 bits. All three
> come from the frame descriptor and none of them is range checked, so a
> device can drive the product up to about 1.1e12 against a type that
> holds 2.1e9.
>
> The kernel builds with -fno-strict-overflow, so this is a defined
> wraparound rather than undefined behaviour, but the stored value is
> still wrong. A descriptor reporting bpp=207, wWidth=18501 and
> wHeight=43738 has a true frame size of 20937965595 and the driver
> stores 27. For bpp=233, wWidth=63435 and wHeight=58989 it stores 0, and
> uvc_queue_setup() then hands vb2 a zero plane size.
>
> Evaluate the product in 64-bit arithmetic by casting the first operand.
>
> Signed-off-by: Natasha Klaus <natalie.klaus@xxxxxxxxxxxxxxxxxxxxxxx>
> ---
> This is RFC because the patch is deliberately incomplete and I would
> rather you pick the remedy than guess at it.
>
> The cast fixes the arithmetic, but dwMaxVideoFrameBufferSize is a
> 32-bit field, so the assignment still truncates and sizes above 4 GiB
> are not representable. The zero case survives too: bpp=32,
> wWidth=32768, wHeight=32768 gives exactly 2^35, the 64-bit quotient is
> exactly 2^32, and the stored value is 0 again.
>
> So the open question is what should happen when a descriptor claims a
> frame that does not fit. As I see it:
>
>   1. this patch, accepting that oversized values truncate
>   2. cap at U32_MAX
>   3. reject the frame descriptor, on the grounds that a device claiming
>      a 100 GB frame is lying
>   4. range check wWidth, wHeight and bpp at parse time, which would
>      also cover the two sibling computations
>
> I am happy to send whichever you prefer as a proper patch.
>
> Two other places take the same unchecked operands the same way:
> uvc_v4l2_get_bytesperline() in uvc_v4l2.c, and the bandwidth estimate
> in uvc_fixup_video_ctrl() in uvc_video.c, which is clamped only from
> below. If you want those fixed I would send a series rather than fold
> them in here.
>
> How this came up: we are building a memory-safe Rust rewrite of the UVC
> descriptor parser and proving properties about it, and the Rust and the
> C disagreed on this field. Standalone reproducer, one command, builds
> under gcc and clang with and without -fno-strict-overflow:
>
>   https://github.com/runtimeverification/uvc-frame-size-overflow
>
> Two caveats. This was measured on the isolated expression with the
> kernel operand types, not on a running kernel and not on a UVC gadget,
> so the vb2 behaviour above comes from reading the code rather than from
> observing it. And I was unable to search the list archives from my
> environment, so apologies if this has already been discussed.
>
>  drivers/media/usb/uvc/uvc_driver.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index e289cc71ba98..5dfbd487732d 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -297,8 +297,8 @@ static int uvc_parse_frame(struct uvc_device *dev,
>          * the value from the frame size.
>          */
>         if (!(format->flags & UVC_FMT_FLAG_COMPRESSED))
> -               frame->dwMaxVideoFrameBufferSize = format->bpp * frame->wWidth
> -                                                * frame->wHeight / 8;
> +               frame->dwMaxVideoFrameBufferSize =
> +                       (u64)format->bpp * frame->wWidth * frame->wHeight / 8;
>
>         /*
>          * Clamp the default frame interval to the boundaries. A zero
>
> base-commit: 8d3ae59288f1e7d58d76558a6ee96d533bc5019f
> --
> 2.34.1
>
>


-- 
Ricardo Ribalda