In the Linux kernel, the following vulnerability has been resolved: ufs: validate cylinder group metadata before caching it ufs_read_cylin

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:

 1ucpi->c_cgx    = fs32_to_cpu(sb, ucg->cg_cgx);
 2ucpi->c_rotor  = fs32_to_cpu(sb, ucg->cg_rotor);
 3ucpi->c_frotor = fs32_to_cpu(sb, ucg->cg_frotor);
 4ucpi->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.