Re: [PATCH v2] hw/display/qxl: validate primary surface stride against width

[Akihiko Odaki <akihiko.odaki@gmail.com>]

Sorry for a late review; I missed this patch.

On 2026/07/25 21:24, marcandre.lureau@redhat.com wrote:
> From: Marc-André Lureau <marcandre.lureau@redhat.com>
> 
> The existing validation in qxl_create_guest_primary() checks that
> abs(stride) * height fits in vgamem_size and that stride is 4-byte
> aligned, but never checks that abs(stride) is large enough to hold one
> row of pixels for the declared width and format.
> 
> A malicious guest can create a primary surface with a stride much
> smaller than width * bytes_per_pixel (e.g. stride=4 for a 64-wide 32bpp
> surface). The spice server rejects this via red_validate_surface(), but
> the return is void and QEMU unconditionally proceeds to set up the local
> rendering state. On the next display refresh, VNC or SDL reads width *
> bytes_pp per scanline from a region backed by only stride bytes per
> row, causing a host-side out-of-bounds read.
> 
> Add three checks in qxl_create_guest_primary() before creating the
> surface:
>   - reject unknown surface formats
>   - reject zero width or height
>   - reject surfaces where abs(stride) < width * bytes_per_pixel
> 
> Also fix three related issues in qxl-render.c:
>   - qxl_blit() used abs_stride to advance the dst pointer into the
>     DisplaySurface, but when stride is negative the DisplaySurface is a
>     packed buffer whose stride may be smaller. Use surface_stride()
>     instead.
>   - qxl_render_update_area_unlocked() uses guest_head0_width (set via
>     QXL_IO_MONITORS_CONFIG_ASYNC) without validating it against
>     abs_stride, bypassing the new validation. Clamp the effective width
>     to abs_stride / bytes_pp to prevent out-of-bounds access while
>     tolerating the normal transient where the monitor config arrives
>     before the primary surface is resized to match.
>   - Similarly, guest_head0_height bypasses qxl_create_guest_primary()
>     validation. Without clamping, abs_stride * height can overrun
>     vgamem_size, and the product can also overflow 32 bits (e.g.
>     abs_stride=16 MiB, height=256 wraps to zero), defeating the
>     qxl_phys2virt() bounds check. Clamp height to
>     vgamem_size / abs_stride to prevent both.
> 
> Fixes: CVE-2026-16271
> Fixes: 3761abb16784 ("hw/display/qxl: fix signed to unsigned comparison")

The Fixes: 3761abb16784 tag is incorrect. That commit only repaired 
overflow in an existing total-size check; short-stride surfaces were 
already accepted. The vulnerable local renderer originated in 
a19cbfb34642, while the monitor-dimension bypass came from 979f7ef8966b 
and the overflowable qxl_phys2virt() size expression from 
8efec0ef8bbc/6dbbf055148c.

> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3637
> Reported-by: huntr bubble
> Signed-off-by: Marc-Andre Lureau <marcandre.lureau@redhat.com>
> ---
> v2:
>   - also clamp height
>   - factor out qxl_format_bpp() helper
> ---
>   hw/display/qxl.h        |  2 ++
>   hw/display/qxl-render.c | 46 ++++++++++++++++----------------
>   hw/display/qxl.c        | 59 +++++++++++++++++++++++++++++++++++++++++
>   3 files changed, 84 insertions(+), 23 deletions(-)
> 
> diff --git a/hw/display/qxl.h b/hw/display/qxl.h
> index 48d664f77736..6f5b96fe86cc 100644
> --- a/hw/display/qxl.h
> +++ b/hw/display/qxl.h
> @@ -181,6 +181,8 @@ void qxl_spice_oom(PCIQXLDevice *qxl);
>   void qxl_spice_reset_memslots(PCIQXLDevice *qxl);
>   void qxl_spice_reset_image_cache(PCIQXLDevice *qxl);
>   void qxl_spice_reset_cursor(PCIQXLDevice *qxl);
> +bool qxl_format_bpp(PCIQXLDevice *qxl, SpiceSurfaceFmt format,
> +                    uint32_t *bytes_pp, uint32_t *bits_pp);
>   
>   /* qxl-logger.c */
>   int qxl_log_cmd_cursor(PCIQXLDevice *qxl, QXLCursorCmd *cmd, int group_id);
> diff --git a/hw/display/qxl-render.c b/hw/display/qxl-render.c
> index 4799c9e8befd..b0a71a95ad66 100644
> --- a/hw/display/qxl-render.c
> +++ b/hw/display/qxl-render.c
> @@ -27,6 +27,7 @@
>   static void qxl_blit(PCIQXLDevice *qxl, QXLRect *rect)
>   {
>       DisplaySurface *surface = qemu_console_surface(qxl->vga.con);
> +    int dst_stride = surface_stride(surface);
>       uint8_t *dst = surface_data(surface);
>       uint8_t *src;
>       int len, i;
> @@ -45,14 +46,14 @@ static void qxl_blit(PCIQXLDevice *qxl, QXLRect *rect)
>       } else {
>           src += rect->top * qxl->guest_primary.abs_stride;
>       }
> -    dst += rect->top  * qxl->guest_primary.abs_stride;
> +    dst += rect->top  * dst_stride;
>       src += rect->left * qxl->guest_primary.bytes_pp;
>       dst += rect->left * qxl->guest_primary.bytes_pp;
>       len  = (rect->right - rect->left) * qxl->guest_primary.bytes_pp;
>   
>       for (i = rect->top; i < rect->bottom; i++) {
>           memcpy(dst, src, len);
> -        dst += qxl->guest_primary.abs_stride;
> +        dst += dst_stride;
>           src += qxl->guest_primary.qxl_stride;
>       }
>   }
> @@ -64,27 +65,9 @@ void qxl_render_resize(PCIQXLDevice *qxl)
>       qxl->guest_primary.qxl_stride = sc->stride;
>       qxl->guest_primary.abs_stride = abs(sc->stride);
>       qxl->guest_primary.resized++;
> -    switch (sc->format) {
> -    case SPICE_SURFACE_FMT_16_555:
> -        qxl->guest_primary.bytes_pp = 2;
> -        qxl->guest_primary.bits_pp = 15;
> -        break;
> -    case SPICE_SURFACE_FMT_16_565:
> -        qxl->guest_primary.bytes_pp = 2;
> -        qxl->guest_primary.bits_pp = 16;
> -        break;
> -    case SPICE_SURFACE_FMT_32_xRGB:
> -    case SPICE_SURFACE_FMT_32_ARGB:
> -        qxl->guest_primary.bytes_pp = 4;
> -        qxl->guest_primary.bits_pp = 32;
> -        break;
> -    default:
> -        fprintf(stderr, "%s: unhandled format: %x\n", __func__,
> -                qxl->guest_primary.surface.format);
> -        qxl->guest_primary.bytes_pp = 4;
> -        qxl->guest_primary.bits_pp = 32;
> -        break;
> -    }
> +    /* fallback to default bpp if format is unknown */
> +    qxl_format_bpp(qxl, sc->format, &qxl->guest_primary.bytes_pp,
> +                   &qxl->guest_primary.bits_pp);
>   }
>   
>   static void qxl_set_rect_to_surface(PCIQXLDevice *qxl, QXLRect *area)
> @@ -103,6 +86,23 @@ static void qxl_render_update_area_unlocked(PCIQXLDevice *qxl)
>       int height = qxl->guest_head0_height ?: qxl->guest_primary.surface.height;
>       int i;
>   
> +    if (width <= 0 || height <= 0) {
> +        qxl_set_guest_bug(qxl, "%s: invalid dimension %dx%d",
> +                          __func__, width, height);
> +        goto end;
> +    }

The new dimension check runs before the existing !guest_primary.data 
exit. A qxl device starts in QXL_MODE_UNDEFINED with a zeroed primary 
surface, and qxl_render_update() explicitly calls this function in that 
state. Thus a host refresh—GTK, VNC, or screendump—sees 0×0 and calls 
qxl_set_guest_bug().

Below is an LLM-generated reproducer:

qemu-system-x86_64 -machine q35 -nodefaults -device qxl,id=qxl0 \
-display none -S -qmp stdio -trace enable=qxl_set_guest_bug <<'EOF'
{"execute":"qmp_capabilities"}
{"execute":"screendump","arguments":{"filename":"/dev/null","device":"qxl0","format":"ppm"}}
{"execute":"quit"}
EOF

Regards,
Akihiko Odaki

> +
> +    if (qxl->guest_primary.bytes_pp > 0) {
> +        int max_width = qxl->guest_primary.abs_stride
> +                        / qxl->guest_primary.bytes_pp;
> +        width = MIN(width, max_width);
> +    }
> +
> +    if (qxl->guest_primary.abs_stride > 0) {
> +        int max_height = qxl->vgamem_size / qxl->guest_primary.abs_stride;
> +        height = MIN(height, max_height);
> +    }
> +
>       if (qxl->guest_primary.resized) {
>           qxl->guest_primary.resized = 0;
>           qxl->guest_primary.data = qxl_phys2virt(qxl,
> diff --git a/hw/display/qxl.c b/hw/display/qxl.c
> index b7d871b9ee33..384b8767b8e6 100644
> --- a/hw/display/qxl.c
> +++ b/hw/display/qxl.c
> @@ -1489,6 +1489,47 @@ static void qxl_create_guest_primary_complete(PCIQXLDevice *qxl)
>       qxl_render_resize(qxl);
>   }
>   
> +/*
> + * Convert a SpiceSurfaceFormat to bytes per pixel and bits per pixel.
> + *
> + * Only valid for surface suitable for rendering.
> + */
> +bool qxl_format_bpp(PCIQXLDevice *qxl, SpiceSurfaceFmt format,
> +                    uint32_t *bytes_pp, uint32_t *bits_pp)
> +{
> +    uint32_t bypp = 4;
> +    uint32_t bipp = 32;
> +    bool ret = true;
> +
> +    switch (format) {
> +    case SPICE_SURFACE_FMT_16_555:
> +        bypp = 2;
> +        bipp = 15;
> +        break;
> +    case SPICE_SURFACE_FMT_16_565:
> +        bypp = 2;
> +        bipp = 16;
> +        break;
> +    case SPICE_SURFACE_FMT_32_xRGB:
> +    case SPICE_SURFACE_FMT_32_ARGB:
> +        bypp = 4;
> +        bipp = 32;
> +        break;
> +    default:
> +        ret = false;
> +        qxl_set_guest_bug(qxl, "%s: unhandled format: %x", __func__, format);
> +    }
> +
> +    if (bytes_pp != NULL) {
> +        *bytes_pp = bypp;
> +    }
> +    if (bits_pp != NULL) {
> +        *bits_pp = bipp;
> +    }
> +
> +    return ret;
> +}
> +
>   static void qxl_create_guest_primary(PCIQXLDevice *qxl, int loadvm,
>                                        qxl_async_io async)
>   {
> @@ -1496,6 +1537,7 @@ static void qxl_create_guest_primary(PCIQXLDevice *qxl, int loadvm,
>       QXLSurfaceCreate *sc = &qxl->guest_primary.surface;
>       uint32_t requested_height = le32_to_cpu(sc->height);
>       int requested_stride = le32_to_cpu(sc->stride);
> +    uint32_t bytes_pp;
>   
>       if (requested_stride == INT32_MIN ||
>           abs(requested_stride) * (uint64_t)requested_height
> @@ -1532,6 +1574,23 @@ static void qxl_create_guest_primary(PCIQXLDevice *qxl, int loadvm,
>           return;
>       }
>   
> +    if (!qxl_format_bpp(qxl, surface.format, &bytes_pp, NULL)) {
> +        return;
> +    }
> +
> +    if (surface.width == 0 || surface.height == 0) {
> +        qxl_set_guest_bug(qxl, "%s: zero dimension %ux%u",
> +                          __func__, surface.width, surface.height);
> +        return;
> +    }
> +
> +    if ((uint64_t)surface.width * bytes_pp > abs(surface.stride)) {
> +        qxl_set_guest_bug(qxl, "%s: stride too small for width:"
> +                          " stride %d width %u bpp %u",
> +                          __func__, surface.stride, surface.width, bytes_pp);
> +        return;
> +    }
> +
>       surface.mouse_mode = true;
>       surface.group_id   = MEMSLOT_GROUP_GUEST;
>       if (loadvm) {