sysvipc/sem: mitigate semnum index against spectre v1

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>

1 files changed, 14 insertions, 4 deletions

diff --git a/ipc/sem.c b/ipc/sem.c
index 59a3cd1d3252..5af1943ad782 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -85,6 +85,7 @@
 #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"
@@ -368,6 +369,7 @@ 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 */
@@ -385,7 +387,8 @@ 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,
@@ -638,7 +641,8 @@ 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;
 
@@ -718,7 +722,9 @@ 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;
 
@@ -1356,6 +1362,7 @@ 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);
@@ -1509,6 +1516,8 @@ 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) {
@@ -2081,7 +2090,8 @@ static long do_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) {