[PATCH] proc: fix null pointer deref in proc_pid_permission()

[Xiaotian Feng <xtfeng@gmail.com>]

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

[ 1870.574045] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
[ 1870.574065] IP: [<ffffffff81217d34>] proc_pid_permission+0x64/0xe0
[ 1870.574088] PGD 112075067 PUD 112814067 PMD 0
[ 1870.574106] Oops: 0002 [#1] PREEMPT SMP

This is a regression introduced by commit 0499680a, kernel should
return -ESRCH if get_proc_task() failed.

Signed-off-by: Xiaotian Feng <dannyfeng@tencent.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Vasiliy Kulikov <segoon@openwall.com>
Cc: Stephen Wilson <wilsons@start.ca>
Cc: David Rientjes <rientjes@google.com>
---
 fs/proc/base.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 8173dfd..5485a53 100644
--- a/fs/proc/base.c
+++ b/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);
 
-- 
1.7.5.4