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

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

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

pciwritelegacyio() 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 pcilegacywrite(bus, off, (u32 )buf, count);

kernfsfopwriteiter() allocates the buffer with kmalloc(len + 1), so a 1-byte write to the legacyio 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 getunalignedle16() and getunalignedle32() 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 outle16() and outle32() 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 legacyio file is root-only and exists only on Alpha and PowerPC, the two architectures that define HAVEPCILEGACY.