apparmor: fix missing bounds check on DEFAULT table in verify_dfa()

The verify_dfa() function only checks DEFAULT_TABLE bounds when the state
is not differentially encoded.

When the verification loop traverses the differential encoding chain,
it reads k = DEFAULT_TABLE[j] and uses k as an array index without
validation. A malformed DFA with DEFAULT_TABLE[j] >= state_count,
therefore, causes both out-of-bounds reads and writes.

[   57.179855] ==================================================================
[   57.180549] BUG: KASAN: slab-out-of-bounds in verify_dfa+0x59a/0x660
[   57.180904] Read of size 4 at addr ffff888100eadec4 by task su/993

[   57.181554] CPU: 1 UID: 0 PID: 993 Comm: su Not tainted 6.19.0-rc7-next-20260127 #1 PREEMPT(lazy)
[   57.181558] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   57.181563] Call Trace:
[   57.181572]  <TASK>
[   57.181577]  dump_stack_lvl+0x5e/0x80
[   57.181596]  print_report+0xc8/0x270
[   57.181605]  ? verify_dfa+0x59a/0x660
[   57.181608]  kasan_report+0x118/0x150
[   57.181620]  ? verify_dfa+0x59a/0x660
[   57.181623]  verify_dfa+0x59a/0x660
[   57.181627]  aa_dfa_unpack+0x1610/0x1740
[   57.181629]  ? __kmalloc_cache_noprof+0x1d0/0x470
[   57.181640]  unpack_pdb+0x86d/0x46b0
[   57.181647]  ? srso_alias_return_thunk+0x5/0xfbef5
[   57.181653]  ? srso_alias_return_thunk+0x5/0xfbef5
[   57.181656]  ? aa_unpack_nameX+0x1a8/0x300
[   57.181659]  aa_unpack+0x20b0/0x4c30
[   57.181662]  ? srso_alias_return_thunk+0x5/0xfbef5
[   57.181664]  ? stack_depot_save_flags+0x33/0x700
[   57.181681]  ? kasan_save_track+0x4f/0x80
[   57.181683]  ? kasan_save_track+0x3e/0x80
[   57.181686]  ? __kasan_kmalloc+0x93/0xb0
[   57.181688]  ? __kvmalloc_node_noprof+0x44a/0x780
[   57.181693]  ? aa_simple_write_to_buffer+0x54/0x130
[   57.181697]  ? policy_update+0x154/0x330
[   57.181704]  aa_replace_profiles+0x15a/0x1dd0
[   57.181707]  ? srso_alias_return_thunk+0x5/0xfbef5
[   57.181710]  ? __kvmalloc_node_noprof+0x44a/0x780
[   57.181712]  ? aa_loaddata_alloc+0x77/0x140
[   57.181715]  ? srso_alias_return_thunk+0x5/0xfbef5
[   57.181717]  ? _copy_from_user+0x2a/0x70
[   57.181730]  policy_update+0x17a/0x330
[   57.181733]  profile_replace+0x153/0x1a0
[   57.181735]  ? rw_verify_area+0x93/0x2d0
[   57.181740]  vfs_write+0x235/0xab0
[   57.181745]  ksys_write+0xb0/0x170
[   57.181748]  do_syscall_64+0x8e/0x660
[   57.181762]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   57.181765] RIP: 0033:0x7f6192792eb2

Remove the MATCH_FLAG_DIFF_ENCODE condition to validate all DEFAULT_TABLE
entries unconditionally.

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>
Signed-off-by: Massimiliano Pellizzer <massimiliano.pellizzer@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/security/apparmor/match.c b/security/apparmor/match.c
index 30c46b119869..2aad2cbed565 100644
--- a/security/apparmor/match.c
+++ b/security/apparmor/match.c
@@ -160,9 +160,10 @@ static int verify_dfa(struct aa_dfa *dfa)
 	if (state_count == 0)
 		goto out;
 	for (i = 0; i < state_count; i++) {
-		if (!(BASE_TABLE(dfa)[i] & MATCH_FLAG_DIFF_ENCODE) &&
-		    (DEFAULT_TABLE(dfa)[i] >= state_count))
+		if (DEFAULT_TABLE(dfa)[i] >= state_count) {
+			pr_err("AppArmor DFA default state out of bounds");
 			goto out;
+		}
 		if (BASE_TABLE(dfa)[i] & MATCH_FLAGS_INVALID) {
 			pr_err("AppArmor DFA state with invalid match flags");
 			goto out;