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Subjectmm: 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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