CVE-2021-36084 (C, published 2021-07-01T03:15Z)
Description: value: The CIL compiler in SELinux 3.2 has a use-after-free in __cil_verify_classperms (called from __cil_verify_classpermission and __cil_pre_verify_helper).
Severity: LOW (CVSS v3 base score: 3.3)
Weakness: CWE-416 \u2014 Use After Free
The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
Repository: https://github.com/SELinuxProject/selinux
Files changed:
libsepol/cil/src/cil_reset_ast.c
Commit message: libsepol/cil: Destroy classperms list when resetting classpermission

Nicolas Iooss reports:
  A few months ago, OSS-Fuzz found a crash in the CIL compiler, which
  got reported as
  https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28648 (the title
  is misleading, or is caused by another issue that conflicts with the
  one I report in this message). Here is a minimized CIL policy which
  reproduces the issue:

  (class CLASS (PERM))
  (classorder (CLASS))
  (sid SID)
  (sidorder (SID))
  (user USER)
  (role ROLE)
  (type TYPE)
  (category CAT)
  (categoryorder (CAT))
  (sensitivity SENS)
  (sensitivityorder (SENS))
  (sensitivitycategory SENS (CAT))
  (allow TYPE self (CLASS (PERM)))
  (roletype ROLE TYPE)
  (userrole USER ROLE)
  (userlevel USER (SENS))
  (userrange USER ((SENS)(SENS (CAT))))
  (sidcontext SID (USER ROLE TYPE ((SENS)(SENS))))

  (classpermission CLAPERM)

  (optional OPT
      (roletype nonexistingrole nonexistingtype)
      (classpermissionset CLAPERM (CLASS (PERM)))
  )

  The CIL policy fuzzer (which mimics secilc built with clang Address
  Sanitizer) reports:

  ==36541==ERROR: AddressSanitizer: heap-use-after-free on address
  0x603000004f98 at pc 0x56445134c842 bp 0x7ffe2a256590 sp
  0x7ffe2a256588
  READ of size 8 at 0x603000004f98 thread T0
      #0 0x56445134c841 in __cil_verify_classperms
  /selinux/libsepol/src/../cil/src/cil_verify.c:1620:8
      #1 0x56445134a43e in __cil_verify_classpermission
  /selinux/libsepol/src/../cil/src/cil_verify.c:1650:9
      #2 0x56445134a43e in __cil_pre_verify_helper
  /selinux/libsepol/src/../cil/src/cil_verify.c:1715:8
      #3 0x5644513225ac in cil_tree_walk_core
  /selinux/libsepol/src/../cil/src/cil_tree.c:272:9
      #4 0x564451322ab1 in cil_tree_walk
  /selinux/libsepol/src/../cil/src/cil_tree.c:316:7
      #5 0x5644513226af in cil_tree_walk_core
  /selinux/libsepol/src/../cil/src/cil_tree.c:284:9
      #6 0x564451322ab1 in cil_tree_walk
  /selinux/libsepol/src/../cil/src/cil_tree.c:316:7
      #7 0x5644512b88fd in cil_pre_verify
  /selinux/libsepol/src/../cil/src/cil_post.c:2510:7
      #8 0x5644512b88fd in cil_post_process
  /selinux/libsepol/src/../cil/src/cil_post.c:2524:7
      #9 0x5644511856ff in cil_compile
  /selinux/libsepol/src/../cil/src/cil.c:564:7

The classperms list of a classpermission rule is created and filled
in when classpermissionset rules are processed, so it doesn't own any
part of the list and shouldn't retain any of it when it is reset.

Destroy the classperms list (without destroying the data in it)  when
resetting a classpermission rule.

Reported-by: Nicolas Iooss <nicolas.iooss@m4x.org>
Signed-off-by: James Carter <jwcart2@gmail.com>


Vulnerable code:
	cil_reset_classperms_list(cp->classperms);

Fixed code:
	cil_list_destroy(&cp->classperms, CIL_FALSE);