apparmor: fix differential encoding verification

commit 39440b137546a3aa383cfdabc605fb73811b6093 upstream.

Differential encoding allows loops to be created if it is abused. To
prevent this the unpack should verify that a diff-encode chain
terminates.

Unfortunately the differential encode verification had two bugs.

1. it conflated states that had gone through check and already been
   marked, with states that were currently being checked and marked.
   This means that loops in the current chain being verified are treated
   as a chain that has already been verified.

2. the order bailout on already checked states compared current chain
   check iterators j,k instead of using the outer loop iterator i.
   Meaning a step backwards in states in the current chain verification
   was being mistaken for moving to an already verified state.

Move to a double mark scheme where already verified states get a
different mark, than the current chain being kept. This enables us
to also drop the backwards verification check that was the cause of
the second error as any already verified state is already marked.

BUG=None
TEST=presubmit
RELEASE_NOTE=Fixed the "CrackArmor" vulnerability in the Linux kernel.

cos-patch: security-high
Fixes: 031dcc8f4e84 ("apparmor: dfa add support for state differential encoding")
Reported-by: Qualys Security Advisory <qsa@qualys.com>
Tested-by: Salvatore Bonaccorso <carnil@debian.org>
Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Reviewed-by: Cengiz Can <cengiz.can@canonical.com>
Change-Id: I9dfaee7a1c4378889c09ea074a39ec4dd985e4fe
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 34fc60b125ed1d4eb002c76b0664bf0619492167)
Signed-off-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/137849
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: He Gao <hegao@google.com>