CVE-2026-64074: fs/statmount: fix slab out-of-bounds write in statmount_mnt_idmap

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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

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

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

fs/statmount: fix slab out-of-bounds write in statmount_mnt_idmap

statmount_mnt_idmap() writes one mapping with seq_printf() and then
manually advances seq->count to include the NUL separator.

If seq_printf() overflows, seq_set_overflow() sets seq->count to
seq->size. The manual seq->count++ changes this to seq->size + 1.
seq_has_overflowed() then no longer detects the overflow. The corrupted
count returns to statmount_string(), which later executes:

    seq->buf[seq->count++] = '\0';

This causes a 1-byte NULL out-of-bounds write on the dynamically
allocated seq buffer.

Fix this by checking for overflow immediately after seq_printf().

The Linux kernel CVE team has assigned CVE-2026-64074 to this issue.


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

	Issue introduced in 6.15 with commit 37c4a9590e1efcae7749682239fc22a330d2d325 and fixed in 6.18.34 with commit e37ea2c6f17f273813ea4e8e94c102591d598ce1
	Issue introduced in 6.15 with commit 37c4a9590e1efcae7749682239fc22a330d2d325 and fixed in 7.0.11 with commit 93614949dc86f068e3c32c32cf1ee2a2323177a7
	Issue introduced in 6.15 with commit 37c4a9590e1efcae7749682239fc22a330d2d325 and fixed in 7.1 with commit a3bf0f28d4ba16e1f35f8c983bb04426b87e2a78

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-2026-64074
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:
	fs/mnt_idmapping.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/e37ea2c6f17f273813ea4e8e94c102591d598ce1
	https://git.kernel.org/stable/c/93614949dc86f068e3c32c32cf1ee2a2323177a7
	https://git.kernel.org/stable/c/a3bf0f28d4ba16e1f35f8c983bb04426b87e2a78