CVE-2026-89722

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.