Re: [PATCH 2/3] media: uvcvideo: Fix integer overflow in frame buffer size calculation

Hi Natasha

On Thu, 20 Aug 2026 at 11:57, Natasha Klaus
<[email protected]> wrote:
>
> From: Noam Ben Shimon <[email protected]>
>
> In the function uvc_parse_frame(), it recomputes
> dwMaxVideoFrameBufferSize for uncompressed formats. This helps working
> around devices that report it wrong:
>
>         frame->dwMaxVideoFrameBufferSize = format->bpp * frame->wWidth
>                                          * frame->wHeight / 8;
>
> These three arguments originate from the device's own descriptors, and
> therefore can be decided by it. bpp is a u8 and wWidth and wHeight are
> u16. The expression is evaluated in int, and the maximum value is
> 255 * 65535 * 65535 (which is roughly 510 times INT_MAX).
> A device that declares large dimensions therefore overflows a signed
> int here.
> The kernel is built using -fno-strict-overflow, so this wraps rather
> than being miscompiled, but the wrapped value (which is often negative)
> is then divided by 8 and stored in a u32 used as a size.
>
> Two examples for this (using legal field values):
>
>   - 32 bpp, 16384x4096: the product is exactly 2^31 and wraps to
>     INT_MIN. After division and conversion to u32 the field has the
>     value 4026531840 rather than 268435456.
>
>   - 16 bpp, 16384x16384: the product is exactly 2^32 and wraps to 0.
>     The field holds 0, rather than the correct 536870912.
>
> I don't think memory corruption is a consequence of this. The value
> reaches uvc_queue_setup() as the vb2 buffer size, and every copy on the
> decode path is bounded by buf->length, which uvc_buffer_prepare() gets
> from vb2_plane_size() rather than this field. What a wrapped value does
> instead is make the driver describe the stream inconsistently.
> For an uncompressed format uvc_fixup_video_ctrl() copies it into
> ctrl->dwMaxVideoFrameSize unconditionally, and that becomes the
> sizeimage reported by VIDIOC_G_FMT. This is while width, height and
> bytesperline continue to describe the full frame.
> This also makes uvc_video_validate_buffer() mark error on all frames,
> because it is comparing bytesused against the same number.
>
> Compute the size in 64-bit, and if the result does not fit in the u32
> field then skip the frame descriptor. An uncompressed frame this large
> is probably not a real device, and skipping it leaves the rest of the
> format and the streaming interface usable.
>
> The rounding also changes from truncation to round-up. Truncation is
> pre-existing rather than introduced here: the original expression used
> integer division, so it has rounded a partial trailing byte away since
> the driver was merged. Rounding up is the right direction for a buffer
> size, and DIV_ROUND_UP() against BITS_PER_BYTE is what the rest of the
> media tree uses for this computation, including uvc_parse_format()
> itself for the FORCE_BPP quirk.
>
> Fixes: c0efd232929c ("V4L/DVB (8145a): USB Video Class driver")
> Cc: [email protected]
> Signed-off-by: Noam Ben Shimon <[email protected]>
> Signed-off-by: Natasha Klaus <[email protected]>
> ---
> Changes from Noam's v2:
> - Rebased onto patch 1/3; this patch no longer applies standalone.
> - Diagnostic changed from uvc_dbg(dev, DESCR, ...) to dev_warn() on
>   &streaming->intf->dev, reworded to begin "UVC non compliance: " and to
>   say the frame is skipped. The explicit device and interface numbers are
>   dropped from the message text because dev_warn() already identifies the
>   interface. (The original could not be kept as-is in any case: patch 1/3
>   removes the local alts variable it referenced.)
> - bpp, wWidth and wHeight added to the message text (David Laight), matching
>   the wording of the diagnostic in 3/3.
> - The >> 3 replaced with DIV_ROUND_UP() against BITS_PER_BYTE (David Laight),
>   so a partial trailing byte is no longer dropped. The overflow check is
>   applied to the rounded-up value.
> - The return value is still -EINVAL; what changed is its meaning, which
>   patch 1/3 redefines as "skip this frame descriptor" rather than "fail the
>   whole streaming interface".
> - Last paragraph of the commit message reworded from "reject the frame
>   descriptor ... rejection is consistent with the other checks over
>   malformed-descriptors in this function" to describe skipping instead, and
>   a paragraph added on the rounding change.
> - Ricardo Ribalda's Reviewed-by dropped, as it was given on the unmodified
>   v2.
> - Submitter's Signed-off-by added.
> - Fixes: and Cc: stable lines unchanged.
>
> Rounding up also moves the U32_MAX boundary: two inputs within the field
> limits (bpp=79 at 10077x43161 and bpp=237 at 3359x43161) landed exactly on
> U32_MAX with the old truncation and are now rejected, since the rounded-up
> size is not representable.
>
> Build tested on x86_64 only. gcc-multilib was not available in my
> environment, so the 32-bit code generation for the DIV_ROUND_UP() on a u64
> was not verified; the divisor is the power-of-two constant BITS_PER_BYTE, so
> I expect a shift rather than a libgcc 64-bit division helper, but I have not
> confirmed it. No hardware and no UVC gadget were used.
>
>  drivers/media/usb/uvc/uvc_driver.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/media/usb/uvc/uvc_driver.c b/drivers/media/usb/uvc/uvc_driver.c
> index 0cc0e351d139..eb7177ea291d 100644
> --- a/drivers/media/usb/uvc/uvc_driver.c
> +++ b/drivers/media/usb/uvc/uvc_driver.c
> @@ -295,9 +295,21 @@ static int uvc_parse_frame(struct uvc_device *dev,
>          * information. For uncompressed formats this can be fixed by computing
>          * the value from the frame size.
>          */
> -       if (!(format->flags & UVC_FMT_FLAG_COMPRESSED))
> -               frame->dwMaxVideoFrameBufferSize = format->bpp * frame->wWidth
> -                                                * frame->wHeight / 8;
> +       if (!(format->flags & UVC_FMT_FLAG_COMPRESSED)) {
> +               u64 bufsize;
> +
> +               bufsize = DIV_ROUND_UP((u64)format->bpp * frame->wWidth *
> +                                      frame->wHeight, BITS_PER_BYTE);

I believe this should be DIV_ROUND_UP_ULL. Eventhough BITS_PER_BYTE is
a power-of-2, some compilers might have brain damage and complain
about 64 bit division.

> +               if (bufsize > U32_MAX) {
> +                       dev_warn(&streaming->intf->dev,
> +                                "UVC non compliance: FRAME %u computed buffer size overflows (%ux%u, %u bpp), skipping it.\n",
> +                                frame->bFrameIndex, frame->wWidth,
> +                                frame->wHeight, format->bpp);
> +                       return -EINVAL;
> +               }
> +
> +               frame->dwMaxVideoFrameBufferSize = bufsize;
> +       }
>
>         /*
>          * Clamp the default frame interval to the boundaries. A zero
> --
> 2.34.1
>
With that change, feel free to add my:

Reviewed-by: Ricardo Ribalda <[email protected]>

-- 
Ricardo Ribalda