apparmor: fix side-effect bug in match_char() macro usage

commit 8756b68 upstream.

The match_char() macro evaluates its character parameter multiple
times when traversing differential encoding chains. When invoked
with *str++, the string pointer advances on each iteration of the
inner do-while loop, causing the DFA to check different characters
at each iteration and therefore skip input characters.
This results in out-of-bounds reads when the pointer advances past
the input buffer boundary.

[   94.984676] ==================================================================
[   94.985301] BUG: KASAN: slab-out-of-bounds in aa_dfa_match+0x5ae/0x760
[   94.985655] Read of size 1 at addr ffff888100342000 by task file/976

[   94.986319] CPU: 7 UID: 1000 PID: 976 Comm: file Not tainted 6.19.0-rc7-next-20260127 #1 PREEMPT(lazy)
[   94.986322] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014
[   94.986329] Call Trace:
[   94.986341]  <TASK>
[   94.986347]  dump_stack_lvl+0x5e/0x80
[   94.986374]  print_report+0xc8/0x270
[   94.986384]  ? aa_dfa_match+0x5ae/0x760
[   94.986388]  kasan_report+0x118/0x150
[   94.986401]  ? aa_dfa_match+0x5ae/0x760
[   94.986405]  aa_dfa_match+0x5ae/0x760
[   94.986408]  __aa_path_perm+0x131/0x400
[   94.986418]  aa_path_perm+0x219/0x2f0
[   94.986424]  apparmor_file_open+0x345/0x570
[   94.986431]  security_file_open+0x5c/0x140
[   94.986442]  do_dentry_open+0x2f6/0x1120
[   94.986450]  vfs_open+0x38/0x2b0
[   94.986453]  ? may_open+0x1e2/0x2b0
[   94.986466]  path_openat+0x231b/0x2b30
[   94.986469]  ? __x64_sys_openat+0xf8/0x130
[   94.986477]  do_file_open+0x19d/0x360
[   94.986487]  do_sys_openat2+0x98/0x100
[   94.986491]  __x64_sys_openat+0xf8/0x130
[   94.986499]  do_syscall_64+0x8e/0x660
[   94.986515]  ? count_memcg_events+0x15f/0x3c0
[   94.986526]  ? srso_alias_return_thunk+0x5/0xfbef5
[   94.986540]  ? handle_mm_fault+0x1639/0x1ef0
[   94.986551]  ? vma_start_read+0xf0/0x320
[   94.986558]  ? srso_alias_return_thunk+0x5/0xfbef5
[   94.986561]  ? srso_alias_return_thunk+0x5/0xfbef5
[   94.986563]  ? fpregs_assert_state_consistent+0x50/0xe0
[   94.986572]  ? srso_alias_return_thunk+0x5/0xfbef5
[   94.986574]  ? arch_exit_to_user_mode_prepare+0x9/0xb0
[   94.986587]  ? srso_alias_return_thunk+0x5/0xfbef5
[   94.986588]  ? irqentry_exit+0x3c/0x590
[   94.986595]  entry_SYSCALL_64_after_hwframe+0x76/0x7e
[   94.986597] RIP: 0033:0x7fda4a79c3ea

Fix by extracting the character value before invoking match_char,
ensuring single evaluation per outer loop.

Fixes: 074c1cd ("apparmor: dfa move character match into a macro")
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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: m-pellizzer

security/apparmor/match.c

@@ -408,13 +408,18 @@ aa_state_t aa_dfa_match_len(struct aa_dfa *dfa, aa_state_t start,
 	if (dfa->tables[YYTD_ID_EC]) {
 		/* Equivalence class table defined */
 		u8 *equiv = EQUIV_TABLE(dfa);
-		for (; len; len--)
-			match_char(state, def, base, next, check,
-				   equiv[(u8) *str++]);
+		for (; len; len--) {
+			u8 c = equiv[(u8) *str];
+
+			match_char(state, def, base, next, check, c);
+			str++;
+		}
 	} else {
 		/* default is direct to next state */
-		for (; len; len--)
-			match_char(state, def, base, next, check, (u8) *str++);
+		for (; len; len--) {
+			match_char(state, def, base, next, check, (u8) *str);
+			str++;
+		}
 	}
 
 	return state;
@@ -448,13 +453,18 @@ aa_state_t aa_dfa_match(struct aa_dfa *dfa, aa_state_t start, const char *str)
 		/* Equivalence class table defined */
 		u8 *equiv = EQUIV_TABLE(dfa);
 		/* default is direct to next state */
-		while (*str)
-			match_char(state, def, base, next, check,
-				   equiv[(u8) *str++]);
+		while (*str) {
+			u8 c = equiv[(u8) *str];
+
+			match_char(state, def, base, next, check, c);
+			str++;
+		}
 	} else {
 		/* default is direct to next state */
-		while (*str)
-			match_char(state, def, base, next, check, (u8) *str++);
+		while (*str) {
+			match_char(state, def, base, next, check, (u8) *str);
+			str++;
+		}
 	}
 
 	return state;