From a3cc0069e151e5eb5db57bb4e86b00861d5b97ab Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= Date: Fri, 10 Jul 2026 17:43:52 +0400 Subject: [PATCH] hw/display/qxl: fix TOCTOU in cursor chunk data_size handling MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Snapshot chunk.data_size into a host-local variable before passing it to qxl_phys2virt() for validation, and pass it through qxl_cursor() and qxl_unpack_chunks() so that no subsequent code re-reads the field. Without this, a racing vCPU can inflate data_size between the qxl_phys2virt() validation and the memcpy in qxl_unpack_chunks(), causing a source read past the validated region. In practice the read stays within the guest's own VRAM mmap, so the impact is limited. Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3757 Reported-by: Feifan Qian Signed-off-by: Marc-Andre Lureau Reviewed-by: Philippe Mathieu-Daudé Message-ID: <20260710134352.2313675-1-marcandre.lureau@redhat.com> Signed-off-by: Philippe Mathieu-Daudé --- hw/display/qxl-render.c | 28 +++++++++++++++++----------- 1 file changed, 17 insertions(+), 11 deletions(-) diff --git a/hw/display/qxl-render.c b/hw/display/qxl-render.c index 3bf634ee05..4799c9e8be 100644 --- a/hw/display/qxl-render.c +++ b/hw/display/qxl-render.c @@ -217,7 +217,8 @@ void qxl_render_update_area_done(PCIQXLDevice *qxl, QXLCookie *cookie) } static void qxl_unpack_chunks(void *dest, size_t size, PCIQXLDevice *qxl, - QXLDataChunk *chunk, uint32_t group_id) + QXLDataChunk *chunk, uint32_t group_id, + uint32_t chunk_data_size) { uint32_t max_chunks = 32; size_t offset = 0; @@ -225,22 +226,21 @@ static void qxl_unpack_chunks(void *dest, size_t size, PCIQXLDevice *qxl, QXLPHYSICAL next_chunk_phys = 0; for (;;) { - bytes = MIN(size - offset, chunk->data_size); + bytes = MIN(size - offset, chunk_data_size); memcpy(dest + offset, chunk->data, bytes); offset += bytes; if (offset == size) { return; } next_chunk_phys = chunk->next_chunk; - /* fist time, only get the next chunk's data size */ chunk = qxl_phys2virt(qxl, next_chunk_phys, group_id, sizeof(QXLDataChunk)); if (!chunk) { return; } - /* second time, check data size and get data */ + chunk_data_size = chunk->data_size; chunk = qxl_phys2virt(qxl, next_chunk_phys, group_id, - sizeof(QXLDataChunk) + chunk->data_size); + sizeof(QXLDataChunk) + chunk_data_size); if (!chunk) { return; } @@ -252,7 +252,7 @@ static void qxl_unpack_chunks(void *dest, size_t size, PCIQXLDevice *qxl, } static QEMUCursor *qxl_cursor(PCIQXLDevice *qxl, QXLCursor *cursor, - uint32_t group_id) + uint32_t group_id, uint32_t chunk_data_size) { QEMUCursor *c; uint8_t *and_mask, *xor_mask; @@ -272,11 +272,11 @@ static QEMUCursor *qxl_cursor(PCIQXLDevice *qxl, QXLCursor *cursor, case SPICE_CURSOR_TYPE_MONO: /* Assume that the full cursor is available in a single chunk. */ size = 2 * cursor_get_mono_bpl(c) * c->height; - if (size != cursor->data_size || cursor->chunk.data_size < size) { + if (size != cursor->data_size || chunk_data_size < size) { qxl_set_guest_bug(qxl, "%s: bad monochrome cursor %ux%u" " data_size %u chunk_size %u", __func__, c->width, c->height, - cursor->data_size, cursor->chunk.data_size); + cursor->data_size, chunk_data_size); goto fail; } and_mask = cursor->chunk.data; @@ -288,7 +288,8 @@ static QEMUCursor *qxl_cursor(PCIQXLDevice *qxl, QXLCursor *cursor, break; case SPICE_CURSOR_TYPE_ALPHA: size = sizeof(uint32_t) * c->width * c->height; - qxl_unpack_chunks(c->data, size, qxl, &cursor->chunk, group_id); + qxl_unpack_chunks(c->data, size, qxl, &cursor->chunk, group_id, + chunk_data_size); if (qxl->debug > 2) { cursor_print_ascii_art(c, "qxl/alpha"); } @@ -325,19 +326,23 @@ int qxl_render_cursor(PCIQXLDevice *qxl, QXLCommandExt *ext) } switch (cmd->type) { case QXL_CURSOR_SET: + { + uint32_t chunk_data_size; + /* First read the QXLCursor to get QXLDataChunk::data_size ... */ cursor = qxl_phys2virt(qxl, cmd->u.set.shape, ext->group_id, sizeof(QXLCursor)); if (!cursor) { return 1; } + chunk_data_size = cursor->chunk.data_size; /* Then read including the chunked data following QXLCursor. */ cursor = qxl_phys2virt(qxl, cmd->u.set.shape, ext->group_id, - sizeof(QXLCursor) + cursor->chunk.data_size); + sizeof(QXLCursor) + chunk_data_size); if (!cursor) { return 1; } - c = qxl_cursor(qxl, cursor, ext->group_id); + c = qxl_cursor(qxl, cursor, ext->group_id, chunk_data_size); if (c == NULL) { c = cursor_builtin_left_ptr(); } @@ -351,6 +356,7 @@ int qxl_render_cursor(PCIQXLDevice *qxl, QXLCommandExt *ext) qemu_mutex_unlock(&qxl->ssd.lock); qemu_bh_schedule(qxl->ssd.cursor_bh); break; + } case QXL_CURSOR_MOVE: qemu_mutex_lock(&qxl->ssd.lock); qxl->ssd.mouse_x = cmd->u.position.x;