RE: [PATCH v5 2/2] drm/hyperv: validate VMBus packet size in receive callback
From: Michael Kelley <hidden>
Date: 2026-05-23 15:17:46
Also in:
dri-devel, lkml
From: Berkant Koc <redacted> Sent: Saturday, May 23, 2026 6:28 AM
hyperv_receive_sub() reads msg->vid_hdr.type and dispatches into one
of four message-type branches without knowing how many bytes the host
wrote into hv->recv_buf. The completion path then runs
memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE), so the consumer that
wakes on wait_for_completion_timeout() can read up to 16 KiB of
residue from a prior message as if it were the response payload.
Pass bytes_recvd into hyperv_receive_sub() and reject any packet that
does not cover the pipe + synthvid header. A single switch on
msg->vid_hdr.type then computes the type-specific payload size: the
three completion-driving types (SYNTHVID_VERSION_RESPONSE,
SYNTHVID_RESOLUTION_RESPONSE, SYNTHVID_VRAM_LOCATION_ACK) fall through
to a shared exit that requires that size before memcpy/complete, while
SYNTHVID_FEATURE_CHANGE validates its own payload and returns before
reading is_dirt_needed. Unknown types are dropped.
SYNTHVID_RESOLUTION_RESPONSE is variable length: the host fills
resolution_count entries, not the full SYNTHVID_MAX_RESOLUTION_COUNT
array. Validate the fixed prefix first so resolution_count can be
read, bound it against the array, then require only the count-sized
array, so the shorter responses the host actually sends are accepted.
Only run the sub-handler when vmbus_recvpacket() returned success. The
memcpy length is bytes_recvd, which is bounded by VMBUS_MAX_PACKET_SIZE
only on a successful receive; on -ENOBUFS vmbus_recvpacket() instead
reports the required length, which can exceed hv->recv_buf, so copying
bytes_recvd would read and write past the 16 KiB buffers. Gating on the
success return keeps the copy bounded. The nonzero-return path is itself
a malformed-message case and is now logged rather than silently skipped;
channel recovery is not attempted.
Rejected packets are reported via drm_err_ratelimited() rather than
silently dropped, matching the CoCo-hardened pattern in
hv_kvp_onchannelcallback().
Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <redacted>
Assisted-by: Claude:claude-opus-4-7 berkoc-pipelineThis looks good now. The error checking and reporting is robust and the code is well-structured. Thanks for putting up with my sometimes picky feedback. :-) I also ran a basic smoke-test on my local Hyper-V instance. I can confirm that no error messages or failure were generated in the "good" case where the messages from Hyper-V are properly formatted and sized. I did not simulate bad messages and ensure they are detected. Reviewed-by: Michael Kelley <redacted> Tested-by: Michael Kelley <redacted>
quoted hunk ↗ jump to hunk
--- drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 100 +++++++++++++++++++--- 1 file changed, 87 insertions(+), 13 deletions(-)diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.cb/drivers/gpu/drm/hyperv/hyperv_drm_proto.c index c3d0ff229..4e6f703a1 100644--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c +++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c@@ -420,30 +420,92 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev) return 0; } -static void hyperv_receive_sub(struct hv_device *hdev) +static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd) { struct hyperv_drm_device *hv = hv_get_drvdata(hdev); struct synthvid_msg *msg; + size_t hdr_size; + size_t need; if (!hv) return; - msg = (struct synthvid_msg *)hv->recv_buf; - - /* Complete the wait event */ - if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE || - msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE || - msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) { - memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE); - complete(&hv->wait); + hdr_size = sizeof(struct pipe_msg_hdr) + + sizeof(struct synthvid_msg_hdr); + if (bytes_recvd < hdr_size) { + drm_err_ratelimited(&hv->dev, + "synthvid packet too small for header: %u\n", + bytes_recvd); return; } - if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) { + msg = (struct synthvid_msg *)hv->recv_buf; + need = hdr_size; + + switch (msg->vid_hdr.type) { + case SYNTHVID_VERSION_RESPONSE: + need += sizeof(struct synthvid_version_resp); + break; + case SYNTHVID_RESOLUTION_RESPONSE: + /* + * The resolution response is variable length: the host + * fills resolution_count entries, not the full + * SYNTHVID_MAX_RESOLUTION_COUNT array. Require the fixed + * prefix first so resolution_count can be read, then + * demand exactly the count-sized array. + */ + need += offsetof(struct synthvid_supported_resolution_resp, + supported_resolution); + if (bytes_recvd < need) + break; + if (msg->resolution_resp.resolution_count > + SYNTHVID_MAX_RESOLUTION_COUNT) { + drm_err_ratelimited(&hv->dev, + "synthvid resolution count too large: %u\n", + msg->resolution_resp.resolution_count); + return; + } + need += msg->resolution_resp.resolution_count * + sizeof(struct hvd_screen_info); + break; + case SYNTHVID_VRAM_LOCATION_ACK: + need += sizeof(struct synthvid_vram_location_ack); + break; + case SYNTHVID_FEATURE_CHANGE: + /* + * Not a completion-driving message: validate its own payload + * and consume it here rather than falling through to the + * memcpy/complete shared by the wait-event responses. + */ + if (bytes_recvd < need + + sizeof(struct synthvid_feature_change)) { + drm_err_ratelimited(&hv->dev, + "synthvid feature change packet too small: %u\n", + bytes_recvd); + return; + } hv->dirt_needed = msg->feature_chg.is_dirt_needed; if (hv->dirt_needed) hyperv_hide_hw_ptr(hv->hdev); + return; + default: + return; + } + + /* + * Shared completion path for the wait-event responses + * (VERSION_RESPONSE, RESOLUTION_RESPONSE, VRAM_LOCATION_ACK): + * require the type-specific payload before handing the buffer to + * the waiter. + */ + if (bytes_recvd < need) { + drm_err_ratelimited(&hv->dev, + "synthvid packet too small for type %u: %u < %zu\n", + msg->vid_hdr.type, bytes_recvd, need); + return; } + memcpy(hv->init_buf, msg, bytes_recvd); + complete(&hv->wait); } static void hyperv_receive(void *ctx)@@ -464,9 +526,21 @@ static void hyperv_receive(void *ctx) ret = vmbus_recvpacket(hdev->channel, recv_buf, VMBUS_MAX_PACKET_SIZE, &bytes_recvd, &req_id); - if (bytes_recvd > 0 && - recv_buf->pipe_hdr.type == PIPE_MSG_DATA) - hyperv_receive_sub(hdev); + if (ret) { + /* + * A nonzero return (e.g. -ENOBUFS for an oversized + * packet) is itself a malformed message: bytes_recvd + * then reports the required length rather than a copied + * payload, so it must not be forwarded to the + * sub-handler. Channel recovery is not attempted. + */ + drm_err_ratelimited(&hv->dev, + "vmbus_recvpacket failed: %d (need %u)\n", + ret, bytes_recvd); + } else if (bytes_recvd > 0 && + recv_buf->pipe_hdr.type == PIPE_MSG_DATA) { + hyperv_receive_sub(hdev, bytes_recvd); + } } while (bytes_recvd > 0 && ret == 0); } --2.47.3