CVE-2026-64527: drm/hyperv: validate VMBus packet size in receive callback

From: Greg Kroah-Hartman <[email protected]>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

drm/hyperv: validate VMBus packet size in receive callback

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().

The Linux kernel CVE team has assigned CVE-2026-64527 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 5.15.210 with commit 57d5d697642e05d5dd2d40660817765943dd709f
	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 6.1.176 with commit f5251226551bfec98c4705641b6f94ff1f238d91
	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 6.6.143 with commit 049a6b474823049fe60212f25f26e4b30f44ee8f
	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 6.12.93 with commit 588c84b461393ff1998ac7b97b04f953f642e0df
	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 6.18.35 with commit 164dc7bf17609340233c6bf4f66bb7c7008a0511
	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 7.0.12 with commit c8974d96b6a5496f33dc69a3ce28a7bf5078def4
	Issue introduced in 5.14 with commit 76c56a5affeba1e163b66b9d8cc192e6154466f0 and fixed in 7.1 with commit 7f87763f47a3c22fb50265a00619ef10f2394b18

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64527
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/gpu/drm/hyperv/hyperv_drm_proto.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/57d5d697642e05d5dd2d40660817765943dd709f
	https://git.kernel.org/stable/c/f5251226551bfec98c4705641b6f94ff1f238d91
	https://git.kernel.org/stable/c/049a6b474823049fe60212f25f26e4b30f44ee8f
	https://git.kernel.org/stable/c/588c84b461393ff1998ac7b97b04f953f642e0df
	https://git.kernel.org/stable/c/164dc7bf17609340233c6bf4f66bb7c7008a0511
	https://git.kernel.org/stable/c/c8974d96b6a5496f33dc69a3ce28a7bf5078def4
	https://git.kernel.org/stable/c/7f87763f47a3c22fb50265a00619ef10f2394b18