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Date:	Mon, 22 Dec 2014 10:04:02 -0500
From:	Sasha Levin <sasha.levin@...cle.com>
To:	"linux-mm@...ck.org" <linux-mm@...ck.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Dave Jones <davej@...hat.com>
Subject: mm: NULL ptr deref in unlink_file_vma

Hi all,

While fuzzing with trinity inside a KVM tools guest running the latest -next
kernel, I've stumbled on the following spew:

[  432.376425] BUG: unable to handle kernel NULL pointer dereference at 0000000000000038
[  432.378876] IP: down_write (./arch/x86/include/asm/rwsem.h:105 ./arch/x86/include/asm/rwsem.h:121 kernel/locking/rwsem.c:71)
[  432.380085] PGD 57e5e0067 PUD 57e5e1067 PMD 0
[  432.380085] Oops: 0002 [#1] PREEMPT SMP KASAN
[  432.380085] Dumping ftrace buffer:
[  432.380085]    (ftrace buffer empty)
[  432.380085] Modules linked in:
[  432.380085] CPU: 4 PID: 9249 Comm: trinity-subchil Not tainted 3.18.0-next-20141219-sasha-00047-gaab33f6-dirty #1627
[  432.380085] task: ffff8806a88c8000 ti: ffff880664f3c000 task.ti: ffff880664f3c000
[  432.380085] RIP: down_write (./arch/x86/include/asm/rwsem.h:105 ./arch/x86/include/asm/rwsem.h:121 kernel/locking/rwsem.c:71)
[  432.380085] RSP: 0018:ffff880664f3fc98  EFLAGS: 00010292
[  432.380085] RAX: 0000000000000038 RBX: ffff880101aa7c00 RCX: 1ffff10020354f8f
[  432.380085] RDX: ffffffff00000001 RSI: 1ffff100fe326200 RDI: 0000000000000038
[  432.380085] RBP: ffff880664f3fcb8 R08: 0000000000000000 R09: ffff880101aa6258
[  432.380085] R10: 0000000000000000 R11: 0000000000000000 R12: ffff880000025900
[  432.380085] R13: 0000000000000038 R14: 0000000000000000 R15: ffff880101aa7c00
[  432.380085] FS:  00007f21149c4700(0000) GS:ffff880216400000(0000) knlGS:0000000000000000
[  432.380085] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[  432.380085] CR2: 0000000000000038 CR3: 000000057a4f3000 CR4: 00000000000006a0
[  432.380085] Stack:
[  432.380085]  ffff880101aa7c00 ffff880101aa7c78 ffff880101aa6200 ffff880101aa7c00
[  432.380085]  ffff880664f3fce8 ffffffffa1953e9d[  432.400920] CONFIG_KASAN_INLINE enabled
[  432.400923] GPF could be caused by NULL-ptr deref or user memory access

[  432.402566]  ffff880101aa7c00 00007f210f9f3000
[  432.402566]  dfffe90000000000 ffff880101aa4600 ffff880664f3fd48 ffffffffa1937821
[  432.402566] Call Trace:
[  432.402566] unlink_file_vma (mm/mmap.c:264)
[  432.402566] free_pgtables (mm/memory.c:548)
[  432.402566] exit_mmap (mm/mmap.c:2846)
[  432.402566] ? kmem_cache_free (mm/slub.c:2712 mm/slub.c:2721)
[  432.402566] mmput (kernel/fork.c:659)
[  432.402566] do_exit (./arch/x86/include/asm/thread_info.h:164 kernel/exit.c:438 kernel/exit.c:732)
[  432.402566] ? preempt_count_sub (kernel/sched/core.c:2620)
[  432.402566] ? __this_cpu_preempt_check (lib/smp_processor_id.c:63)
[  432.402566] do_group_exit (include/linux/sched.h:775 kernel/exit.c:858)
[  432.402566] SyS_exit_group (kernel/exit.c:886)
[  432.402566] system_call_fastpath (arch/x86/kernel/entry_64.S:423)
[ 432.402566] Code: 79 05 e8 f9 e9 a6 f2 5d c3 0f 1f 80 00 00 00 00 66 66 66 66 90 48 ba 01 00 00 00 ff ff ff ff 55 48 89 f8 48 89 e5 53 48 83 ec 18 <f0> 48 0f c1 10 85 d2 74 05 e8 f7 e9 a6 f2 65 48 8b 1c 25 80 b9
All code
========
   0:	79 05                	jns    0x7
   2:	e8 f9 e9 a6 f2       	callq  0xfffffffff2a6ea00
   7:	5d                   	pop    %rbp
   8:	c3                   	retq
   9:	0f 1f 80 00 00 00 00 	nopl   0x0(%rax)
  10:	66 66 66 66 90       	data32 data32 data32 xchg %ax,%ax
  15:	48 ba 01 00 00 00 ff 	movabs $0xffffffff00000001,%rdx
  1c:	ff ff ff
  1f:	55                   	push   %rbp
  20:	48 89 f8             	mov    %rdi,%rax
  23:	48 89 e5             	mov    %rsp,%rbp
  26:	53                   	push   %rbx
  27:	48 83 ec 18          	sub    $0x18,%rsp
  2b:*	f0 48 0f c1 10       	lock xadd %rdx,(%rax)		<-- trapping instruction
  30:	85 d2                	test   %edx,%edx
  32:	74 05                	je     0x39
  34:	e8 f7 e9 a6 f2       	callq  0xfffffffff2a6ea30
  39:	65                   	gs
  3a:	48                   	rex.W
  3b:	8b                   	.byte 0x8b
  3c:	1c 25                	sbb    $0x25,%al
  3e:	80                   	.byte 0x80
  3f:	b9                   	.byte 0xb9
	...

Code starting with the faulting instruction
===========================================
   0:	f0 48 0f c1 10       	lock xadd %rdx,(%rax)
   5:	85 d2                	test   %edx,%edx
   7:	74 05                	je     0xe
   9:	e8 f7 e9 a6 f2       	callq  0xfffffffff2a6ea05
   e:	65                   	gs
   f:	48                   	rex.W
  10:	8b                   	.byte 0x8b
  11:	1c 25                	sbb    $0x25,%al
  13:	80                   	.byte 0x80
  14:	b9                   	.byte 0xb9
	...
[  432.402566] RIP down_write (./arch/x86/include/asm/rwsem.h:105 ./arch/x86/include/asm/rwsem.h:121 kernel/locking/rwsem.c:71)
[  432.402566]  RSP <ffff880664f3fc98>
[  432.402566] CR2: 0000000000000038


Thanks,
Sasha
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