Re: Smatch check for Spectre stuff

[Davidlohr Bueso]

So both smatch and coverity are complaining about sysvipc sems.
They look legit (if the policy is, as peterz describes: "kill
any speculation on the first load and not worry if it can be
completed with a dependent load/store").

On Thu, 19 Apr 2018, Dan Carpenter wrote:

ipc/sem.c:1359 semctl_setval() warn: potential spectre issue 'sma->sems' (local 
cap)
ipc/sem.c:1512 semctl_main() warn: potential spectre issue 'sma->sems' (local 
cap)

And for this one the below patch already sanitizes semnum to be between
[0, sma->sem_nsems) before calling into count_semcnt().

ipc/sem.c:1096 count_semcnt() warn: potential spectre issue 'sma->sems'
ipc/sem.c:2084 do_semtimedop() warn: potential spectre issue 'sma->sems'
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'

Thanks,
Davidlohr

----8<--------------------------------------------------
[PATCH] 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.

Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
---

ipc/sem.c | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/ipc/sem.c b/ipc/sem.c
index 06be75d9217a..df623dcefc92 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -84,6 +84,7 @@
#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"
@@ -367,6 +368,7 @@ 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 */
@@ -384,7 +386,8 @@ 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,
@@ -637,7 +640,8 @@ 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;

@@ -717,7 +721,9 @@ 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;

@@ -1349,6 +1355,7 @@ 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);
@@ -1502,6 +1509,8 @@ 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) {
@@ -2072,7 +2081,8 @@ 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) {
--
2.13.6