mm: do_shared_fault: fix potential NULL pointer dereference

[  232.869443] BUG: unable to handle kernel NULL pointer dereference at
0000000000000020
[  232.870230] IP: [<mm/page-writeback.c:1612>]
balance_dirty_pages_ratelimited+0x1e/0x150
[  232.870230] PGD 586e1d067 PUD 586e1e067 PMD 0
[  232.870230] Oops: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[  232.870230] Dumping ftrace buffer:
[  232.870230]    (ftrace buffer empty)
[  232.870230] Modules linked in:
[  232.870230] CPU: 36 PID: 9707 Comm: trinity-c36 Tainted: G        W
3.14.0-rc4-next-20140225-sasha-00010-ga117461 torvalds#42
[  232.870230] task: ffff880586dfb000 ti: ffff880586e34000 task.ti:
ffff880586e34000
[  232.870230] RIP: 0010:[<mm/page-writeback.c:1612>]
[<mm/page-writeback.c:1612>]
balance_dirty_pages_ratelimited+0x1e/0x150
[  232.870230] RSP: 0000:ffff880586e35c58  EFLAGS: 00010282
[  232.870230] RAX: 0000000000000000 RBX: ffff880582831361 RCX:
0000000000000007
[  232.870230] RDX: 0000000000000007 RSI: ffff880586dfbcc0 RDI:
ffff880582831361
[  232.870230] RBP: ffff880586e35c78 R08: 0000000000000000 R09:
0000000000000000
[  232.870230] R10: 0000000000000001 R11: 0000000000000001 R12:
00007f58007ee000
[  232.870230] R13: ffff880c8d6d4f70 R14: 0000000000000200 R15:
ffff880c8dcce710
[  232.870230] FS:  00007f58018bb700(0000) GS:ffff880c8e800000(0000)
knlGS:0000000000000000
[  232.870230] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  232.870230] CR2: 0000000000000020 CR3: 0000000586e1c000 CR4:
00000000000006e0
[  232.870230] Stack:
[  232.870230]  ffff880586e35c78 ffff880586e33400 00007f58007ee000
ffff880c8d6d4f70
[  232.870230]  ffff880586e35cd8 ffffffff8127d241 0000000000000001
0000000000000001
[  232.870230]  0000000000000000 ffffea0032337080 0000000080000000
ffff880586e33400
[  232.870230] Call Trace:
[  232.870230]  [<mm/memory.c:3467>] do_shared_fault+0x1a1/0x1f0
[  232.870230]  [<mm/memory.c:3487>] handle_pte_fault+0xc8/0x230
[  232.870230]  [<arch/x86/include/asm/preempt.h:98>] ? delay_tsc+0xea/0x110
[  232.870230]  [<mm/memory.c:3770>] __handle_mm_fault+0x36e/0x3a0
[  232.870230]  [<include/linux/rcupdate.h:829>] ? rcu_read_unlock+0x5d/0x60
[  232.870230]  [<include/linux/memcontrol.h:148>] handle_mm_fault+0x10b/0x1b0
[  232.870230]  [<arch/x86/mm/fault.c:1147>] ? __do_page_fault+0x2e2/0x590
[  232.870230]  [<arch/x86/mm/fault.c:1214>] __do_page_fault+0x551/0x590
[  232.870230]  [<kernel/sched/cputime.c:681>] ? vtime_account_user+0x91/0xa0
[  232.870230]  [<arch/x86/include/asm/atomic.h:26>] ?
context_tracking_user_exit+0xa8/0x1c0
[  232.870230]  [<arch/x86/include/asm/preempt.h:98>] ?
_raw_spin_unlock+0x30/0x50
[  232.870230]  [<kernel/sched/cputime.c:681>] ? vtime_account_user+0x91/0xa0
[  232.870230]  [<arch/x86/include/asm/atomic.h:26>] ?
context_tracking_user_exit+0xa8/0x1c0
[  232.870230]  [<arch/x86/include/asm/atomic.h:26>] do_page_fault+0x3d/0x70
[  232.870230]  [<arch/x86/kernel/kvm.c:263>] do_async_page_fault+0x35/0x100
[  232.870230]  [<arch/x86/kernel/entry_64.S:1496>] async_page_fault+0x28/0x30
[  232.870230] Code: 66 66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 48 83
ec 20 48 89 5d e8 4c
89 65 f0 4c 89 6d f8 48 89 fb 48 8b 87 50 01 00 00 <f6> 40 20 01 0f 85 18 01
00 00 65 48 8b 14 25 40
da 00 00 44 8b
[  232.870230] RIP  [<mm/page-writeback.c:1612>]
balance_dirty_pages_ratelimited+0x1e/0x150
[  232.870230]  RSP <ffff880586e35c58>
[  232.870230] CR2: 0000000000000020

See: https://lkml.org/lkml/2014/2/25/463

This bug was first introduced by commit: 'mm: introduce do_shared_fault()
and drop do_fault()'(3a379a6) The root cause is
'fault_page->mapping' might be set to NULL just after the if condition and
pass down to 'balance_dirty_pages_ratelimited()'.  This patch fix this
issue by use a local variable just as before.

Signed-off-by: Bob Liu <[email protected]>
Reported-by: Sasha Levin <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: aet00

mm/memory.c

@@ -3415,6 +3415,7 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 		pgoff_t pgoff, unsigned int flags, pte_t orig_pte)
 {
 	struct page *fault_page;
+	struct address_space *mapping;
 	spinlock_t *ptl;
 	pte_t entry, *pte;
 	int dirtied = 0;
@@ -3475,13 +3476,14 @@ static int do_shared_fault(struct mm_struct *mm, struct vm_area_struct *vma,
 
 	if (set_page_dirty(fault_page))
 		dirtied = 1;
+	mapping = fault_page->mapping;
 	unlock_page(fault_page);
-	if ((dirtied || vma->vm_ops->page_mkwrite) && fault_page->mapping) {
+	if ((dirtied || vma->vm_ops->page_mkwrite) && mapping) {
 		/*
 		 * Some device drivers do not set page.mapping but still
 		 * dirty their pages
 		 */
-		balance_dirty_pages_ratelimited(fault_page->mapping);
+		balance_dirty_pages_ratelimited(mapping);
 	}
 
 	/* file_update_time outside page_lock */