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.

Cc: stable@vger.kernel.org
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
---
 arch/powerpc/kernel/pci-common.c |  9 ++-------
 drivers/pci/pci-sysfs.c          | 18 +++++++++++++++---
 2 files changed, 17 insertions(+), 10 deletions(-)

diff --git a/arch/powerpc/kernel/pci-common.c b/arch/powerpc/kernel/pci-common.c
index 8efe95a0c4ff..fdc57fa2ece6 100644
--- a/arch/powerpc/kernel/pci-common.c
+++ b/arch/powerpc/kernel/pci-common.c

@@ -626,19 +626,14 @@ int pci_legacy_write(struct pci_bus *bus, loff_t port, u32 val, size_t size)
 		return -ENXIO;
 	addr = hose->io_base_virt + port;
 
-	/* WARNING: The generic code is idiotic. It gets passed a pointer
-	 * to what can be a 1, 2 or 4 byte quantity and always reads that
-	 * as a u32, which means that we have to correct the location of
-	 * the data read within those 32 bits for size 1 and 2
-	 */
 	switch(size) {
 	case 1:
-		out_8(addr, val >> 24);
+		out_8(addr, val);
 		return 1;
 	case 2:
 		if (port & 1)
 			return -EINVAL;
-		out_le16(addr, val >> 16);
+		out_le16(addr, val);
 		return 2;
 	case 4:
 		if (port & 3)

diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
index d37860841260..b56000ba3a33 100644
--- a/drivers/pci/pci-sysfs.c
+++ b/drivers/pci/pci-sysfs.c

@@ -933,12 +933,24 @@ static ssize_t pci_write_legacy_io(struct file *filp, struct kobject *kobj,
 				   char *buf, loff_t off, size_t count)
 {
 	struct pci_bus *bus = to_pci_bus(kobj_to_dev(kobj));
+	u32 val;
 
-	/* Only support 1, 2 or 4 byte accesses */
-	if (count != 1 && count != 2 && count != 4)
+	/* Only support 1, 2 or 4 byte accesses. */
+	switch (count) {
+	case 1:
+		val = *(u8 *)buf;
+		break;
+	case 2:
+		val = get_unaligned_le16(buf);
+		break;
+	case 4:
+		val = get_unaligned_le32(buf);
+		break;
+	default:
 		return -EINVAL;
+	}
 
-	return pci_legacy_write(bus, off, *(u32 *)buf, count);
+	return pci_legacy_write(bus, off, val, count);
 }
 
 /**

-- 
2.54.0