[PATCH] partitions: aix: bound the pp_count scan to the ppe array

[Bryam Vargas <hexlabsecurity@proton.me>]

aix_partition() reads the physical volume descriptor into a fixed-size
struct pvd and then scans its physical-partition-extent array:

	int numpps = be16_to_cpu(pvd->pp_count);
	...
	for (i = 0; i < numpps; i += 1) {
		struct ppe *p = pvd->ppe + i;
		...
		lp_ix = be16_to_cpu(p->lp_ix);

pvd points at a single kmalloc()'d struct pvd whose ppe[] member holds a
fixed ARRAY_SIZE(pvd->ppe) (1016) entries, but the loop runs up to the
on-disk pp_count.  pp_count is an unvalidated __be16 read straight from
the descriptor, so a crafted AIX image with pp_count larger than 1016
drives the loop to read pvd->ppe[i] past the end of the allocation (up to
65535 entries, ~2 MB out of bounds).

The partition scan runs without mounting anything, when a block device
with a crafted AIX/IBM partition table appears (an attacker-supplied
image attached with losetup -P, or a device auto-scanned by udev), via
msdos_partition() -> aix_partition().

Clamp the scan to the number of entries the ppe[] array can hold.

Fixes: 6ceea22bbbc8 ("partitions: add aix lvm partition support files")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
---
Reproduced on v7.1-rc6 with KASAN (CONFIG_PARTITION_ADVANCED +
CONFIG_AIX_PARTITION).  A crafted disk image whose AIX/IBM partition table
sets pp_count to 0xffff, attached with `losetup -fP image.img` (in-kernel
partition scan, no mount), is reported by KASAN:

  BUG: KASAN: slab-out-of-bounds in aix_partition+0xb6e/0xee0
  Read of size 2 at addr ... by task losetup
   aix_partition
   msdos_partition
   bdev_disk_changed
   loop_reread_partitions
   loop_configure
   lo_ioctl
   __x64_sys_ioctl

i.e. a read past the end of the kmalloc(sizeof(struct pvd)) object.  A control
image with pp_count == 1016 (== ARRAY_SIZE(pvd->ppe)) is clean.  With this
patch the crafted image is parsed with no out-of-bounds access.

This is the read-loop sibling of the lvd scan bounded by Michael Bommarito's
"partitions: aix: bound the lvd scan to one sector"; that change does not
touch the pp_count/ppe[] loop, so the two are complementary (separate hunks).

 block/partitions/aix.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/block/partitions/aix.c b/block/partitions/aix.c
index 29b8f4cebb63..f3c4174e003e 100644
--- a/block/partitions/aix.c
+++ b/block/partitions/aix.c
@@ -226,6 +226,15 @@ int aix_partition(struct parsed_partitions *state)
 		int next_lp_ix = 1;
 		int lp_ix;
 
+		/*
+		 * pvd was read into a fixed-size struct pvd whose ppe[] array
+		 * holds ARRAY_SIZE(pvd->ppe) entries.  pp_count is an
+		 * unvalidated on-disk __be16, so clamp the scan to the array
+		 * size to avoid walking past the allocation.
+		 */
+		if (numpps > ARRAY_SIZE(pvd->ppe))
+			numpps = ARRAY_SIZE(pvd->ppe);
+
 		for (i = 0; i < numpps; i += 1) {
 			struct ppe *p = pvd->ppe + i;
 			unsigned int lv_ix;
-- 
2.43.0