Hello,
> 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.
Applied to the sysfs branch. I would like for the 0-day bot to pick this
up, and also to have some linux-next soak time, if possible.
Thank you!
Krzysztof
