apparmor: ensure WB_HISTORY_SIZE value is a power of 2

WB_HISTORY_SIZE was defined to be a value not a power of 2, despite a
comment in the declaration of struct match_workbuf stating it is and a
modular arithmetic usage in the inc_wb_pos macro assuming that it is. Bump
WB_HISTORY_SIZE's value up to 32 and add a BUILD_BUG_ON_NOT_POWER_OF_2
line to ensure that any future changes to the value of WB_HISTORY_SIZE
respect this requirement.

Fixes: 136db99 ("apparmor: increase left match history buffer size")

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: canonical-rlee287

security/apparmor/include/match.h

@@ -137,7 +137,8 @@ aa_state_t aa_dfa_matchn_until(struct aa_dfa *dfa, aa_state_t start,
 
 void aa_dfa_free_kref(struct kref *kref);
 
-#define WB_HISTORY_SIZE 24
+/* This needs to be a power of 2 */
+#define WB_HISTORY_SIZE 32
 struct match_workbuf {
 	unsigned int count;
 	unsigned int pos;

security/apparmor/match.c

@@ -681,6 +681,7 @@ aa_state_t aa_dfa_matchn_until(struct aa_dfa *dfa, aa_state_t start,
 
 #define inc_wb_pos(wb)						\
 do {								\
+	BUILD_BUG_ON_NOT_POWER_OF_2(WB_HISTORY_SIZE);			\
 	wb->pos = (wb->pos + 1) & (WB_HISTORY_SIZE - 1);		\
 	wb->len = (wb->len + 1) & (WB_HISTORY_SIZE - 1);		\
 } while (0)