In the Linux kernel, the following vulnerability has been resolved:
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.
