CVE-2026-64293: iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read

CVE-2026-64293: iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

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

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

iommufd: Use sizeof(*hdr) instead of sizeof(hdr) in veventq read

The bound-check in iommufd_veventq_fops_read() for the normal vEVENT
path uses sizeof(hdr) where the surrounding code uses sizeof(*hdr):

	if (!vevent_for_lost_events_header(cur) &&
	    sizeof(hdr) + cur->data_len > count - done) {

hdr is declared as struct iommufd_vevent_header *, so sizeof(hdr)
evaluates to the size of the pointer.  Surrounding code uses
sizeof(*hdr) consistently:

	if (done >= count || sizeof(*hdr) > count - done) {
	...
	if (copy_to_user(buf + done, hdr, sizeof(*hdr))) {
	...
	done += sizeof(*hdr);

struct iommufd_vevent_header is currently 8 bytes (two __u32 fields,
flags and sequence), so on 64-bit (sizeof(void *) == 8) the two
expressions happen to be equal and the check works as intended.

On 32-bit (sizeof(void *) == 4) the check under-counts the header by
4 bytes: a vEVENT whose data_len causes 8 + cur->data_len to exceed
count - done while 4 + cur->data_len does not will pass the check,
then the loop will copy_to_user 8 bytes of header followed by data_len
bytes of payload, writing past the user-supplied buffer.

It is also a latent bug for any future expansion of struct
iommufd_vevent_header beyond sizeof(void *) on 64-bit; the check
should not depend on the type happening to match the host pointer
width.

Use sizeof(*hdr) to match the rest of the function and the actual
amount that will be copied.

The Linux kernel CVE team has assigned CVE-2026-64293 to this issue.


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

	Issue introduced in 6.15 with commit e36ba5ab808ef6237c3148d469c8238674230e2b and fixed in 6.18.39 with commit 04a177f91160ee18da98f5689482cf0f589ec869
	Issue introduced in 6.15 with commit e36ba5ab808ef6237c3148d469c8238674230e2b and fixed in 7.1.4 with commit 0cdbb97a4dbd69abdd2ab998b4fbc7803d4b0b72
	Issue introduced in 6.15 with commit e36ba5ab808ef6237c3148d469c8238674230e2b and fixed in 7.2-rc1 with commit be93d186ae88a92e7aa77e122d4e661fa57b1e39

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-2026-64293
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/iommu/iommufd/eventq.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/04a177f91160ee18da98f5689482cf0f589ec869
	https://git.kernel.org/stable/c/0cdbb97a4dbd69abdd2ab998b4fbc7803d4b0b72
	https://git.kernel.org/stable/c/be93d186ae88a92e7aa77e122d4e661fa57b1e39