[Patch] proc: check error pointer returned by m_start()

[Amerigo Wang <amwang@redhat.com>]

Anca reported a bug:

[15117.080119] BUG: unable to handle kernel paging request at fffffffffffffff3
[15117.080152] IP: [<ffffffff811b4989>] vma_stop+0x19/0x40

Linus did the initial analysis, and found this was caused
by commit ec6fd8a4355c ("report errors in /proc/*/*map*
sanely"), which replaces NULL with various ERR_PTR() cases.

This is true, that commit changed the return value of m_start(),
which will return an error pointer on failure, but Al forgot
to check the error pointer in m_stop() which will be called
when m_start() fails. This patches fixes it.

Reported-by: Anca Emanuel <anca.emanuel@gmail.com>
Tested-by: Anca Emanuel <anca.emanuel@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: WANG Cong <xiyou.wangcong@gmail.com>

---
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 7c708a4..6b82632 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -90,7 +90,7 @@ static void pad_len_spaces(struct seq_file *m, int len)
 
 static void vma_stop(struct proc_maps_private *priv, struct vm_area_struct *vma)
 {
-	if (vma && vma != priv->tail_vma) {
+	if (vma && !IS_ERR(vma) && vma != priv->tail_vma) {
 		struct mm_struct *mm = vma->vm_mm;
 		up_read(&mm->mmap_sem);
 		mmput(mm);