CVE-2026-74478: um: vector: fix use-after-free in vector_mmsg_rx()

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

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

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

um: vector: fix use-after-free in vector_mmsg_rx()

When vector_mmsg_rx() discards a packet whose overlay header fails
verify_header(), it frees the skb and continues the loop:

	if (header_check < 0) {
		dev_kfree_skb_irq(skb);
		vp->estats.rx_encaps_errors++;
		continue;
	}

The normal and short-packet paths fall through to the bottom of the
loop body, which clears the consumed slot and advances the cursors:

	(*skbuff_vector) = NULL;
	mmsg_vector++;
	skbuff_vector++;

The verify_header() < 0 path skips that via continue, so the freed skb
is left in skbuff_vector[] and the cursors do not advance. The next
iteration reads the same slot, gets the freed skb, and frees it again,
producing a refcount underflow / use-after-free in the RX path.

Discard the slot the same way the other paths do before continuing.

Only transports whose verify_header() can return negative are affected:
GRE and L2TPv3 do so on a cookie/session-id mismatch (raw/tap do not),
so any peer on such a transport can trigger it without authentication.

The Linux kernel CVE team has assigned CVE-2026-74478 to this issue.


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

	Issue introduced in 4.17 with commit 49da7e64f33e80edffb1a9eeb230fa4c3f42dffb and fixed in 6.6.151 with commit 4b9601595e8b6b5d18878cac0aeabc687d241111
	Issue introduced in 4.17 with commit 49da7e64f33e80edffb1a9eeb230fa4c3f42dffb and fixed in 6.12.103 with commit 67d58ab4f2ccf7145f3da07e025735a09c79de1b
	Issue introduced in 4.17 with commit 49da7e64f33e80edffb1a9eeb230fa4c3f42dffb and fixed in 6.18.44 with commit 180ff4c81faf01ec4e06082c9daa7c40518ead89
	Issue introduced in 4.17 with commit 49da7e64f33e80edffb1a9eeb230fa4c3f42dffb and fixed in 7.1.8 with commit 804b681002ead233abf49a3efd681f5468a835f9
	Issue introduced in 4.17 with commit 49da7e64f33e80edffb1a9eeb230fa4c3f42dffb and fixed in 7.2-rc6 with commit af421e9aed3920c7ac88c24daa48606c7112feca

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-2026-74478
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:
	arch/um/drivers/vector_kern.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/4b9601595e8b6b5d18878cac0aeabc687d241111
	https://git.kernel.org/stable/c/67d58ab4f2ccf7145f3da07e025735a09c79de1b
	https://git.kernel.org/stable/c/180ff4c81faf01ec4e06082c9daa7c40518ead89
	https://git.kernel.org/stable/c/804b681002ead233abf49a3efd681f5468a835f9
	https://git.kernel.org/stable/c/af421e9aed3920c7ac88c24daa48606c7112feca