CVE-2026-72098: dm-verity: fix buffer overflow in FEC calculation

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

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

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

dm-verity: fix buffer overflow in FEC calculation

There's a buffer overflow in dm-verity-fec:

if (neras && *neras <= v->fec->roots)
	fio->erasures[(*neras)++] = i;

This allows *neras to reach roots + 1 (the post-increment pushes it past
roots). This value is then passed as no_eras to decode_rs8(). Inside the
RS decoder (lib/reed_solomon/decode_rs.c:113-121), the erasure locator
polynomial loop writes lambda[j] where j can reach nroots + 1 — one
element past the end of lambda[] (which is sized nroots + 1, valid
indices 0..nroots). The out-of-bounds write lands on syn[0], corrupting
the syndrome buffer.

The Linux kernel CVE team has assigned CVE-2026-72098 to this issue.


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

	Issue introduced in 4.5 with commit a739ff3f543afbb4a041c16cd0182c8e8d366e70 and fixed in 6.18.42 with commit 5488d3a69d205e28f74f18857b853aa12e778e66
	Issue introduced in 4.5 with commit a739ff3f543afbb4a041c16cd0182c8e8d366e70 and fixed in 7.1.5 with commit f7990c2b0f08b8841fcd2652d1d7002f5994a7a7
	Issue introduced in 4.5 with commit a739ff3f543afbb4a041c16cd0182c8e8d366e70 and fixed in 7.2-rc3 with commit 31d6e6c0ba8d5a7bd59660035a089307100c5e8e

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-2026-72098
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/md/dm-verity-fec.c
	drivers/md/dm-verity-fec.h


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/5488d3a69d205e28f74f18857b853aa12e778e66
	https://git.kernel.org/stable/c/f7990c2b0f08b8841fcd2652d1d7002f5994a7a7
	https://git.kernel.org/stable/c/31d6e6c0ba8d5a7bd59660035a089307100c5e8e