Re: [PATCH] ui/vnc: validate SetPixelFormat field ranges

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jul 13, 2026 at 02:00:56PM +0400, marcandre.lureau@redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> The VNC SetPixelFormat message carries red/green/blue_max as 16-bit
> values, but PixelFormat stores them as uint8_t. A client sending a
> max value above 255 (e.g. 0x0100) passes the existing non-zero check
> but silently truncates to 0 on assignment, leading to a division by
> zero in the Tight PNG palette path.
> 
> Similarly, the shift values are read as uint8_t from the wire but
> used in left-shift expressions (red_max << red_shift). Shifts >= 32
> are undefined behavior in C for 32-bit operands.
> 
> Add explicit range checks for both: reject the connection if any
> channel max exceeds UINT8_MAX, or any shift is >= 32.
> 
> Fixes: CVE-2026-15578
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3976
> Reported-by: dong ling
> Signed-off-by: Marc-Andre Lureau <marcandre.lureau@redhat.com>
> ---
>  ui/vnc.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

> 
> diff --git a/ui/vnc.c b/ui/vnc.c
> index dfc8262d42a..c4ee4738b28 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -2276,6 +2276,16 @@ static void set_pixel_format(VncState *vs, int bits_per_pixel,
>          return;
>      }
>  
> +    if (red_max > UINT8_MAX || green_max > UINT8_MAX || blue_max > UINT8_MAX) {
> +        vnc_client_error(vs);
> +        return;
> +    }
> +
> +    if (red_shift >= 32 || green_shift >= 32 || blue_shift >= 32) {
> +        vnc_client_error(vs);
> +        return;
> +    }
> +
>      vs->client_pf.rmax = red_max ? red_max : 0xFF;
>      vs->client_pf.rbits = ctpopl(red_max);
>      vs->client_pf.rshift = red_shift;

With regards,
Daniel
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