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c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1785937473; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding; bh=DuazKRPwCQG3gdJEcNwpRFs5Vs2ng54NRKJ3FuTsgTE=; b=a9kAB5UBL3UyDKVrP3sPP5re2TYsEKn7GNcd5CnINt6Lb8TZRZd0osnlso+bowvIcdY0aQ cuCxioyQah+8WJcI/RX8NV33bAIw6LVrnHG+WMkMzjuPYtqnPJTJrhgjsAe+tFn9OVHola xcuOQoLLOyhf1lnGPUYktVUScM3DhOg= X-MC-Unique: shBLKlsPNweEeslWgwLqJw-1 X-Mimecast-MFC-AGG-ID: shBLKlsPNweEeslWgwLqJw_1785937470 From: marcandre.lureau@redhat.com To: qemu-devel@nongnu.org Cc: akihiko.odaki@gmail.com, =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= Subject: [PATCH v4] hw/display/qxl: validate primary surface stride against width Date: Wed, 5 Aug 2026 17:44:24 +0400 Message-ID: <20260805134425.249695-1-marcandre.lureau@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.4 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists1p.gnu.org; Received-SPF: pass client-ip=170.10.133.124; envelope-from=marcandre.lureau@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -29 X-Spam_score: -3.0 X-Spam_bar: --- X-Spam_report: (-3.0 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.852, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1785937521524158500 From: Marc-Andr=C3=A9 Lureau 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=3D4 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=3D16 MiB, height=3D256 wraps to zero), defeating the qxl_phys2virt() bounds check. Clamp height to vgamem_size / abs_stride to prevent both. Fixes: CVE-2026-16271 Fixes: a19cbfb34642 ("spice: add qxl device") Fixes: 979f7ef8966b ("qxl: use guest_monitor_config for local renderer.") Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3637 Reported-by: huntr bubble Signed-off-by: Marc-Andre Lureau --- v4: - do not flag zero dimensions as guest bug in render path, since QXL_MODE_UNDEFINED starts with a zeroed primary surface - clamp negative-stride to the surface height, so qxl_blit() cannot walk before it - tweak commit message --- hw/display/qxl.h | 2 ++ hw/display/qxl-render.c | 49 ++++++++++++++++++---------------- hw/display/qxl.c | 59 +++++++++++++++++++++++++++++++++++++++++ 3 files changed, 87 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); =20 /* 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..57820f211fe6 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 =3D qemu_console_surface(qxl->vga.con); + int dst_stride =3D surface_stride(surface); uint8_t *dst =3D surface_data(surface); uint8_t *src; int len, i; @@ -45,14 +46,14 @@ static void qxl_blit(PCIQXLDevice *qxl, QXLRect *rect) } else { src +=3D rect->top * qxl->guest_primary.abs_stride; } - dst +=3D rect->top * qxl->guest_primary.abs_stride; + dst +=3D rect->top * dst_stride; src +=3D rect->left * qxl->guest_primary.bytes_pp; dst +=3D rect->left * qxl->guest_primary.bytes_pp; len =3D (rect->right - rect->left) * qxl->guest_primary.bytes_pp; =20 for (i =3D rect->top; i < rect->bottom; i++) { memcpy(dst, src, len); - dst +=3D qxl->guest_primary.abs_stride; + dst +=3D dst_stride; src +=3D qxl->guest_primary.qxl_stride; } } @@ -64,27 +65,9 @@ void qxl_render_resize(PCIQXLDevice *qxl) qxl->guest_primary.qxl_stride =3D sc->stride; qxl->guest_primary.abs_stride =3D abs(sc->stride); qxl->guest_primary.resized++; - switch (sc->format) { - case SPICE_SURFACE_FMT_16_555: - qxl->guest_primary.bytes_pp =3D 2; - qxl->guest_primary.bits_pp =3D 15; - break; - case SPICE_SURFACE_FMT_16_565: - qxl->guest_primary.bytes_pp =3D 2; - qxl->guest_primary.bits_pp =3D 16; - break; - case SPICE_SURFACE_FMT_32_xRGB: - case SPICE_SURFACE_FMT_32_ARGB: - qxl->guest_primary.bytes_pp =3D 4; - qxl->guest_primary.bits_pp =3D 32; - break; - default: - fprintf(stderr, "%s: unhandled format: %x\n", __func__, - qxl->guest_primary.surface.format); - qxl->guest_primary.bytes_pp =3D 4; - qxl->guest_primary.bits_pp =3D 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); } =20 static void qxl_set_rect_to_surface(PCIQXLDevice *qxl, QXLRect *area) @@ -103,6 +86,26 @@ static void qxl_render_update_area_unlocked(PCIQXLDevic= e *qxl) int height =3D qxl->guest_head0_height ?: qxl->guest_primary.surface.h= eight; int i; =20 + if (width <=3D 0 || height <=3D 0) { + goto end; + } + + if (qxl->guest_primary.bytes_pp > 0) { + int max_width =3D qxl->guest_primary.abs_stride + / qxl->guest_primary.bytes_pp; + width =3D MIN(width, max_width); + } + + if (qxl->guest_primary.qxl_stride < 0) { + /* qxl_blit() uses the primary height to find the first scanline. = */ + height =3D MIN(height, (int)qxl->guest_primary.surface.height); + } + + if (qxl->guest_primary.abs_stride > 0) { + int max_height =3D qxl->vgamem_size / qxl->guest_primary.abs_strid= e; + height =3D MIN(height, max_height); + } + if (qxl->guest_primary.resized) { qxl->guest_primary.resized =3D 0; qxl->guest_primary.data =3D 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(PCIQXL= Device *qxl) qxl_render_resize(qxl); } =20 +/* + * 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 =3D 4; + uint32_t bipp =3D 32; + bool ret =3D true; + + switch (format) { + case SPICE_SURFACE_FMT_16_555: + bypp =3D 2; + bipp =3D 15; + break; + case SPICE_SURFACE_FMT_16_565: + bypp =3D 2; + bipp =3D 16; + break; + case SPICE_SURFACE_FMT_32_xRGB: + case SPICE_SURFACE_FMT_32_ARGB: + bypp =3D 4; + bipp =3D 32; + break; + default: + ret =3D false; + qxl_set_guest_bug(qxl, "%s: unhandled format: %x", __func__, forma= t); + } + + if (bytes_pp !=3D NULL) { + *bytes_pp =3D bypp; + } + if (bits_pp !=3D NULL) { + *bits_pp =3D 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 *qx= l, int loadvm, QXLSurfaceCreate *sc =3D &qxl->guest_primary.surface; uint32_t requested_height =3D le32_to_cpu(sc->height); int requested_stride =3D le32_to_cpu(sc->stride); + uint32_t bytes_pp; =20 if (requested_stride =3D=3D INT32_MIN || abs(requested_stride) * (uint64_t)requested_height @@ -1532,6 +1574,23 @@ static void qxl_create_guest_primary(PCIQXLDevice *q= xl, int loadvm, return; } =20 + if (!qxl_format_bpp(qxl, surface.format, &bytes_pp, NULL)) { + return; + } + + if (surface.width =3D=3D 0 || surface.height =3D=3D 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_p= p); + return; + } + surface.mouse_mode =3D true; surface.group_id =3D MEMSLOT_GROUP_GUEST; if (loadvm) { --=20 2.55.0