Re: [PATCH 1/2] PCI/sysfs: Fix out-of-bounds read in pci_write_legacy_io()

["Krzysztof Wilczyński" <kwilczynski@kernel.org>]

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