[PATCH] pid: add null pointer check in pid_nr_ns()

[Alakesh Haloi]

There has been at least one occurrence where a null pointer derefernce
panic was seen with following stack trace.

 #0 [ffffff800bcd3800] machine_kexec at ffffff8008095fb4
 #1 [ffffff800bcd3860] __crash_kexec at ffffff8008122a30
 #2 [ffffff800bcd39f0] panic at ffffff80080aa054
 #3 [ffffff800bcd3ae0] die at ffffff800808aee8
 #4 [ffffff800bcd3b20] die_kernel_fault at ffffff8008099520
 #5 [ffffff800bcd3b50] __do_kernel_fault at ffffff8008098e50
 #6 [ffffff800bcd3b80] do_translation_fault at ffffff800809929c
 #7 [ffffff800bcd3b90] do_mem_abort at ffffff8008081204
 #8 [ffffff800bcd3d90] el1_ia at ffffff800808304c
     PC: ffffff80080c20ec  [pid_nr_ns+4]
     LR: ffffff80080c231c  [__task_pid_nr_ns+72]
     SP: ffffff800bcd3da0  PSTATE: 60000005
    X29: ffffff800bcd3da0  X28: ffffffc00691c380  X27: 0000000000000001
    X26: 00000000004ce8e8  X25: 00000000004ce8d0  X24: ffffffc00691c3e0
    X23: ffffffc004e8c000  X22: 0000000000000000  X21: ffffffc00b042ed2
    X20: ffffff800876a4f0  X19: 0000000000000000  X18: 0000000000000000
    X17: 0000000000000001  X16: 0000000000000000  X15: 0000000000000000
    X14: 0000000400000003  X13: 0000000000000008  X12: fefefefefefefeff
    X11: 0000000000000000  X10: 0000007fffffffff   X9: 00000000004ce8b0
     X8: 00000000004ce8b0   X7: 0000000000000000   X6: ffffffc00b042ed2
     X5: ffffffc00b042ed2   X4: 0000000000020008   X3: 53206e69616c702f
     X2: ffffff800876a4f0   X1: ffffff800876a4f0   X0: 53206e69616c702f
 #9 [ffffff800bcd3da0] pid_nr_ns at ffffff80080c20e8

Signed-off-by: Alakesh Haloi <alakesh.haloi@xxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/pid.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index a96bc4bf4f86..3767b9e1431d 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -474,7 +474,7 @@ pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns)
 	struct upid *upid;
 	pid_t nr = 0;
 
-	if (pid && ns->level <= pid->level) {
+	if (pid && ns && ns->level <= pid->level) {
 		upid = &pid->numbers[ns->level];
 		if (upid->ns == ns)
 			nr = upid->nr;
-- 
2.17.1