[PATCH 2/2] ufs: validate cylinder group metadata before caching it

ufs_read_cylinder() copies the cylinder group index and the rotor
positions straight from the on-disk group and caches them without any
check:

	ucpi->c_cgx    = fs32_to_cpu(sb, ucg->cg_cgx);
	ucpi->c_rotor  = fs32_to_cpu(sb, ucg->cg_rotor);
	ucpi->c_frotor = fs32_to_cpu(sb, ucg->cg_frotor);
	ucpi->c_irotor = fs32_to_cpu(sb, ucg->cg_irotor);

They are then used as indices during allocation and free:

  - c_cgx indexes the cylinder summary array as
    UFS_SB(sb)->fs_cs(ucpi->c_cgx), so a value past s_ncg writes a 32
    bit count outside the s_csp allocation.

  - c_frotor becomes a bitmap scan start, start = c_frotor >> 3, and
    then length = ((s_fpg + 7) >> 3) - start. A start beyond the block
    bitmap wraps the unsigned length to a huge value, so ubh_scanc()
    walks far past the cylinder group buffers. c_irotor drives the
    inode bitmap the same way.

A crafted image can set any of these freely, turning an ordinary
allocation into an out of bounds access.

Reject a cylinder group whose recorded index does not match the group
being read, or whose rotors fall outside the group, before the metadata
is cached. Valid filesystems keep cg_cgx equal to the group number and
the rotors within the group, so only malformed images are rejected.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Cc: [email protected]
Signed-off-by: Ali Ahmet Memis <[email protected]>
---
 fs/ufs/cylinder.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/ufs/cylinder.c b/fs/ufs/cylinder.c
index a2813270c..b930ee1cf 100644
--- a/fs/ufs/cylinder.c
+++ b/fs/ufs/cylinder.c
@@ -68,6 +68,16 @@ static bool ufs_read_cylinder(struct super_block *sb,
 	ucpi->c_clustersumoff = fs32_to_cpu(sb, ucg->cg_u.cg_44.cg_clustersumoff);
 	ucpi->c_clusteroff = fs32_to_cpu(sb, ucg->cg_u.cg_44.cg_clusteroff);
 	ucpi->c_nclusterblks = fs32_to_cpu(sb, ucg->cg_u.cg_44.cg_nclusterblks);
+
+	/* these on-disk values become array and bitmap indices */
+	if (ucpi->c_cgx != cgno ||
+	    ucpi->c_rotor >= uspi->s_fpg ||
+	    ucpi->c_frotor >= uspi->s_fpg ||
+	    ucpi->c_irotor >= uspi->s_ipg) {
+		ufs_error(sb, __func__,
+			  "inconsistent metadata in cylinder group %u\n", cgno);
+		goto failed;
+	}
 	UFSD("EXIT\n");
 	return true;
 	
-- 
2.54.0