CVE-2026-89722: PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()

CVE-2026-89722: PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()

[Greg Kroah-Hartman]

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

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

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

PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()

pci_write_legacy_io() loads 4 bytes from the kernfs write buffer
regardless of how many bytes userspace wrote:

  if (count != 1 && count != 2 && count != 4)
          return -EINVAL;

  return pci_legacy_write(bus, off, *(u32 *)buf, count);

kernfs_fop_write_iter() allocates the buffer with kmalloc(len + 1),
so a 1-byte write to the legacy_io sysfs file allocates 2 bytes and
the unconditional u32 load reads up to 2 bytes past the end of the
allocation, which KASAN reports as a slab-out-of-bounds read.
Similarly, a 2-byte write overreads by 1 byte.

Thus, read only the number of bytes requested using get_unaligned_le16()
and get_unaligned_le32() for the 2 and 4 byte cases, interpreting the
buffer as little-endian to match the byte ordering of PCI I/O port
space.

The PowerPC implementation previously compensated for the generic
code's native-endian 32-bit load by shifting the value into place
for the 1 and 2 byte cases.  The shifts were only correct on
big-endian kernels.

On little-endian PowerPC (POWER8 and later), they extracted the wrong
bytes, so a 1-byte write wrote an out-of-bounds byte instead of the
requested value.  On big-endian, the native load also caused out_le16()
and out_le32() to reverse the user's bytes on the wire for 2 and 4 byte
writes.  The little-endian helpers resolve both issues, so the shifts
are removed.

No changes are needed for the Alpha platform.

The legacy_io file is root-only and exists only on Alpha and PowerPC,
the two architectures that define HAVE_PCI_LEGACY.

The Linux kernel CVE team has assigned CVE-2026-89722 to this issue.


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

	Fixed in 6.18.50 with commit e892f05f1f790d5721cd8e3c561ed76da7e08bae
	Fixed in 7.2.4 with commit 7a99e9c7011905734cc2739038f2a224b6be0d1f
	Fixed in 7.3-rc1 with commit dc76258d0132df1d831a5a29758bd448ca9c566e

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-2026-89722
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/powerpc/kernel/pci-common.c
	drivers/pci/pci-sysfs.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/e892f05f1f790d5721cd8e3c561ed76da7e08bae
	https://git.kernel.org/stable/c/7a99e9c7011905734cc2739038f2a224b6be0d1f
	https://git.kernel.org/stable/c/dc76258d0132df1d831a5a29758bd448ca9c566e