[PATCH] Fix NULL ->nsproxy dereference in /proc/*/mounts

/proc/*/mounstats was fixed, all right, but...

To reproduce:

	while true; do
		find /proc -type f 2>/dev/null | xargs cat 1>/dev/null 2>/dev/null;
	done

BUG: unable to handle kernel NULL pointer dereference at virtual address 0000000c
 printing eip:
c01754df
*pde = 00000000
Oops: 0000 [#28]
Modules linked in: af_packet ohci_hcd e1000 ehci_hcd uhci_hcd usbcore xfs
CPU:    0
EIP:    0060:[<c01754df>]    Not tainted VLI
EFLAGS: 00010286   (2.6.20-rc5 #1)
EIP is at mounts_open+0x1c/0xac
eax: 00000000   ebx: d5898ac0   ecx: d1d27b18   edx: d1d27a50
esi: e6083e10   edi: d3c87f38   ebp: d5898ac0   esp: d3c87ef0
ds: 007b   es: 007b   ss: 0068
Process cat (pid: 18071, ti=d3c86000 task=f7d5f070 task.ti=d3c86000)
Stack: d5898ac0 e6083e10 d3c87f38 c01754c3 c0147c91 c18c52c0 d343f314 d5898ac0
       00008000 d3c87f38 ffffff9c c0147e09 d5898ac0 00000000 00000000 c0147e4b
       00000000 d3c87f38 d343f314 c18c52c0 c015e53e 00001000 08051000 00000101
Call Trace:
 [<c01754c3>] mounts_open+0x0/0xac
 [<c0147c91>] __dentry_open+0xa1/0x18c
 [<c0147e09>] nameidata_to_filp+0x31/0x3a
 [<c0147e4b>] do_filp_open+0x39/0x40
 [<c015e53e>] seq_read+0x128/0x2aa
 [<c0147e8c>] do_sys_open+0x3a/0x6d
 [<c0147efa>] sys_open+0x1c/0x20
 [<c0102b76>] sysenter_past_esp+0x5f/0x85
 [<c02a0033>] unix_stream_recvmsg+0x3bf/0x4bf
 =======================
Code: 5d c3 89 d8 e8 06 e0 f9 ff eb bd 0f 0b eb fe 55 57 56 53 89 d5 8b 40 f0 31 d2 e8 02 c1 fa ff 89 c2 85 c0 74 5c 8b 80 48 04 00 00 <8b> 58 0c 85 db 74 02 ff 03 ff 4a 08 0f 94 c0 84 c0 75 74 85 db
EIP: [<c01754df>] mounts_open+0x1c/0xac SS:ESP 0068:d3c87ef0

A race with do_exit()'s call to exit_namespaces().
Signed-off-by: NAlexey Dobriyan <adobriyan@openvz.org>
Acked-by: NSerge Hallyn <serue@us.ibm.com>
Signed-off-by: NAndrew Morton <akpm@osdl.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -371,9 +371,11 @@ static int mounts_open(struct inode *inode, struct file *file)
 
 	if (task) {
 		task_lock(task);
-		ns = task->nsproxy->mnt_ns;
-		if (ns)
-			get_mnt_ns(ns);
+		if (task->nsproxy) {
+			ns = task->nsproxy->mnt_ns;
+			if (ns)
+				get_mnt_ns(ns);
+		}
 		task_unlock(task);
 		put_task_struct(task);
 	}