proc: fix null pointer deref in proc_pid_permission()

get_proc_task() can fail to search the task and return NULL,
put_task_struct() will then bomb the kernel with following oops:

  BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
  IP: [<ffffffff81217d34>] proc_pid_permission+0x64/0xe0
  PGD 112075067 PUD 112814067 PMD 0
  Oops: 0002 [raspberrypi#1] PREEMPT SMP

This is a regression introduced by commit 0499680 ("procfs: add hidepid=
and gid= mount options").  The kernel should return -ESRCH if
get_proc_task() failed.

Signed-off-by: Xiaotian Feng <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Vasiliy Kulikov <[email protected]>
Cc: Stephen Wilson <[email protected]>
Acked-by: David Rientjes <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

fs/proc/base.c

@@ -654,6 +654,8 @@ static int proc_pid_permission(struct inode *inode, int mask)
 	bool has_perms;
 
 	task = get_proc_task(inode);
+	if (!task)
+		return -ESRCH;
 	has_perms = has_pid_permissions(pid, task, 1);
 	put_task_struct(task);