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

Overview

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.

Affected

• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < e892f05f1f790d5721cd8e3c561ed76da7e08bae
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < 7a99e9c7011905734cc2739038f2a224b6be0d1f
• Linux >= 1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 < dc76258d0132df1d831a5a29758bd448ca9c566e
• Linux 2.6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Moderate · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4 · no fix planned: Red Hat Enterprise Linux 6, Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, … · updated 2026-09-15 · vex