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c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1784815161; 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=UOt41VTg1gIMOePjbxCy0gpoQUucgBbb1swuQn2CO7A=; b=JwrvDEd10t3VvHY0vhNshWIXz8l4+AtaagD4092/MD7g9NyrNIIQ9cP4Vqfh37fetTN1tn YNWgnHbY/8e7RKWFFdzjsd3l6ZNx4Thk1WnYTDxL2/xitNUVLVVlJqiqXiLkri/cLfaOp8 x+JUwteFHYi3HLOHwWwQuqK+xjI/8G0= X-MC-Unique: nZ3utG7wP_aXSLLdQ2SdNg-1 X-Mimecast-MFC-AGG-ID: nZ3utG7wP_aXSLLdQ2SdNg_1784815157 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 v2] hw/display/qxl: validate primary surface stride against width Date: Thu, 23 Jul 2026 17:59:09 +0400 Message-ID: <20260723135909.429714-1-marcandre.lureau@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 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: permerror client-ip=170.10.129.124; envelope-from=marcandre.lureau@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: 3 X-Spam_score: 0.3 X-Spam_bar: / X-Spam_report: (0.3 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.951, 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_H2=0.001, RCVD_IN_SBL_CSS=3.335, SPF_HELO_PASS=-0.001, T_SPF_PERMERROR=0.01 autolearn=no 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: 1784815199359158500 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 two 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. Fixes: CVE-2026-16271 Fixes: 3761abb16784 ("hw/display/qxl: fix signed to unsigned comparison") Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3637 Reported-by: huntr bubble Signed-off-by: Marc-Andre Lureau --- hw/display/qxl-render.c | 11 +++++++++-- hw/display/qxl.c | 29 +++++++++++++++++++++++++++++ 2 files changed, 38 insertions(+), 2 deletions(-) diff --git a/hw/display/qxl-render.c b/hw/display/qxl-render.c index 4799c9e8befd..231cad17ee3e 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; } } @@ -103,6 +104,12 @@ static void qxl_render_update_area_unlocked(PCIQXLDevi= ce *qxl) int height =3D qxl->guest_head0_height ?: qxl->guest_primary.surface.h= eight; int i; =20 + 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.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..41673c369bca 100644 --- a/hw/display/qxl.c +++ b/hw/display/qxl.c @@ -1496,6 +1496,7 @@ 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 +1533,34 @@ static void qxl_create_guest_primary(PCIQXLDevice *q= xl, int loadvm, return; } =20 + switch (surface.format) { + case SPICE_SURFACE_FMT_16_555: + case SPICE_SURFACE_FMT_16_565: + bytes_pp =3D 2; + break; + case SPICE_SURFACE_FMT_32_xRGB: + case SPICE_SURFACE_FMT_32_ARGB: + bytes_pp =3D 4; + break; + default: + qxl_set_guest_bug(qxl, "%s: unhandled format %d", + __func__, surface.format); + 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