From: Davidlohr Bueso <dave@stgolabs.net>
Subject: sysvipc/sem: mitigate semnum index against spectre v1
Git-commit: ec67aaa46dce26d671b46c94ac674ad0b67d044c
Patch-mainline: v4.18
References: bsc#1192802
Acked-by: Jiri Bohac <jbohac@suse.cz>

Both smatch and coverity are reporting potential issues with spectre
variant 1 with the 'semnum' index within the sma->sems array, ie:

  ipc/sem.c:388 sem_lock() warn: potential spectre issue 'sma->sems'
  ipc/sem.c:641 perform_atomic_semop_slow() warn: potential spectre issue 'sma->sems'
  ipc/sem.c:721 perform_atomic_semop() warn: potential spectre issue 'sma->sems'

Avoid any possible speculation by using array_index_nospec() thus
ensuring the semnum value is bounded to [0, sma->sem_nsems).  With the
exception of sem_lock() all of these are slowpaths.

Link: http://lkml.kernel.org/r/20180423171131.njs4rfm2yzyeg6do@linux-n805
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/ipc/sem.c b/ipc/sem.c
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -83,6 +83,7 @@
 #include <linux/nsproxy.h>
 #include <linux/ipc_namespace.h>
 #include <linux/sched/wake_q.h>
+#include <linux/nospec.h>
 
 #include <linux/uaccess.h>
 #include "util.h"
@@ -329,6 +330,7 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
 			      int nsops)
 {
 	struct sem *sem;
+	int idx;
 
 	if (nsops != 1) {
 		/* Complex operation - acquire a full lock */
@@ -346,7 +348,8 @@ static inline int sem_lock(struct sem_array *sma, struct sembuf *sops,
 	 *
 	 * Both facts are tracked by use_global_mode.
 	 */
-	sem = &sma->sems[sops->sem_num];
+	idx = array_index_nospec(sops->sem_num, sma->sem_nsems);
+	sem = &sma->sems[idx];
 
 	/*
 	 * Initial check for use_global_lock. Just an optimization,
@@ -604,7 +607,8 @@ static int perform_atomic_semop_slow(struct sem_array *sma, struct sem_queue *q)
 	un = q->undo;
 
 	for (sop = sops; sop < sops + nsops; sop++) {
-		curr = &sma->sems[sop->sem_num];
+		int idx = array_index_nospec(sop->sem_num, sma->sem_nsems);
+		curr = &sma->sems[idx];
 		sem_op = sop->sem_op;
 		result = curr->semval;
 
@@ -684,7 +688,9 @@ static int perform_atomic_semop(struct sem_array *sma, struct sem_queue *q)
 	 * until the operations can go through.
 	 */
 	for (sop = sops; sop < sops + nsops; sop++) {
-		curr = &sma->sems[sop->sem_num];
+		int idx = array_index_nospec(sop->sem_num, sma->sem_nsems);
+
+		curr = &sma->sems[idx];
 		sem_op = sop->sem_op;
 		result = curr->semval;
 
@@ -1324,6 +1330,7 @@ static int semctl_setval(struct ipc_namespace *ns, int semid, int semnum,
 		return -EIDRM;
 	}
 
+	semnum = array_index_nospec(semnum, sma->sem_nsems);
 	curr = &sma->sems[semnum];
 
 	ipc_assert_locked_object(&sma->sem_perm);
@@ -1477,6 +1484,8 @@ static int semctl_main(struct ipc_namespace *ns, int semid, int semnum,
 		err = -EIDRM;
 		goto out_unlock;
 	}
+
+	semnum = array_index_nospec(semnum, nsems);
 	curr = &sma->sems[semnum];
 
 	switch (cmd) {
@@ -1934,7 +1943,8 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
 	 */
 	if (nsops == 1) {
 		struct sem *curr;
-		curr = &sma->sems[sops->sem_num];
+		int idx = array_index_nospec(sops->sem_num, sma->sem_nsems);
+		curr = &sma->sems[idx];
 
 		if (alter) {
 			if (sma->complex_count) {
-- 
2.32.0