9p: fix off by one causing access violations and memory corruption

p9_release_pages() would attempt to dereference one value past the end of
pages[]. This would cause the following crashes:

[ 6293.171817] BUG: unable to handle kernel paging request at ffff8807c96f3000
[ 6293.174146] IP: [<ffffffff8412793b>] p9_release_pages+0x3b/0x60
[ 6293.176447] PGD 79c5067 PUD 82c1e3067 PMD 82c197067 PTE 80000007c96f3060
[ 6293.180060] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[ 6293.180060] Modules linked in:
[ 6293.180060] CPU: 62 PID: 174043 Comm: modprobe Tainted: G        W    3.10.0-next-20130710-sasha #3954
[ 6293.180060] task: ffff8807b803b000 ti: ffff880787dde000 task.ti: ffff880787dde000
[ 6293.180060] RIP: 0010:[<ffffffff8412793b>]  [<ffffffff8412793b>] p9_release_pages+0x3b/0x60
[ 6293.214316] RSP: 0000:ffff880787ddfc28  EFLAGS: 00010202
[ 6293.214316] RAX: 0000000000000001 RBX: ffff8807c96f2ff8 RCX: 0000000000000000
[ 6293.222017] RDX: ffff8807b803b000 RSI: 0000000000000001 RDI: ffffea001c7e3d40
[ 6293.222017] RBP: ffff880787ddfc48 R08: 0000000000000000 R09: 0000000000000000
[ 6293.222017] R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000001
[ 6293.222017] R13: 0000000000000001 R14: ffff8807cc50c070 R15: ffff8807cc50c070
[ 6293.222017] FS:  00007f572641d700(0000) GS:ffff8807f3600000(0000) knlGS:0000000000000000
[ 6293.256784] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[ 6293.256784] CR2: ffff8807c96f3000 CR3: 00000007c8e81000 CR4: 00000000000006e0
[ 6293.256784] Stack:
[ 6293.256784]  ffff880787ddfcc8 ffff880787ddfcc8 0000000000000000 ffff880787ddfcc8
[ 6293.256784]  ffff880787ddfd48 ffffffff84128be8 ffff880700000002 0000000000000001
[ 6293.256784]  ffff8807b803b000 ffff880787ddfce0 0000100000000000 0000000000000000
[ 6293.256784] Call Trace:
[ 6293.256784]  [<ffffffff84128be8>] p9_virtio_zc_request+0x598/0x630
[ 6293.256784]  [<ffffffff8115c610>] ? wake_up_bit+0x40/0x40
[ 6293.256784]  [<ffffffff841209b1>] p9_client_zc_rpc+0x111/0x3a0
[ 6293.256784]  [<ffffffff81174b78>] ? sched_clock_cpu+0x108/0x120
[ 6293.256784]  [<ffffffff84122a21>] p9_client_read+0xe1/0x2c0
[ 6293.256784]  [<ffffffff81708a90>] v9fs_file_read+0x90/0xc0
[ 6293.256784]  [<ffffffff812bd073>] vfs_read+0xc3/0x130
[ 6293.256784]  [<ffffffff811a78bd>] ? trace_hardirqs_on+0xd/0x10
[ 6293.256784]  [<ffffffff812bd5a2>] SyS_read+0x62/0xa0
[ 6293.256784]  [<ffffffff841a1a00>] tracesys+0xdd/0xe2
[ 6293.256784] Code: 66 90 48 89 fb 41 89 f5 48 8b 3f 48 85 ff 74 29 85 f6 74 25 45 31 e4 66 0f 1f 84 00 00 00 00 00 e8 eb 14 12 fd 41 ff c4 49 63 c4 <48> 8b 3c c3 48 85 ff 74 05 45 39 e5 75 e7 48 83 c4 08 5b 41 5c
[ 6293.256784] RIP  [<ffffffff8412793b>] p9_release_pages+0x3b/0x60
[ 6293.256784]  RSP <ffff880787ddfc28>
[ 6293.256784] CR2: ffff8807c96f3000
[ 6293.256784] ---[ end trace 50822ee72cd360fc ]---
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/9p/trans_common.c

@@ -24,11 +24,11 @@
  */
 void p9_release_pages(struct page **pages, int nr_pages)
 {
-	int i = 0;
-	while (pages[i] && nr_pages--) {
-		put_page(pages[i]);
-		i++;
-	}
+	int i;
+
+	for (i = 0; i < nr_pages; i++)
+		if (pages[i])
+			put_page(pages[i]);
 }
 EXPORT_SYMBOL(p9_release_pages);