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Subject: Re: kvm: use-after-free in complete_emulated_mmio
To: Wanpeng Li <kernellwp@gmail.com>, Dmitry Vyukov <dvyukov@google.com>
References: <CACT4Y+bdNG23ECLoyEgAOESTEv0wgeBk=VvJQDOt=PNZxBGY7g@mail.gmail.com>
 <CANRm+CyhkB10nAs1yWtso4wuUGmDfx=gyv1Mbemgtqd6iLVZCg@mail.gmail.com>
Cc: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        KVM list <kvm@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Steve Rutherford <srutherford@google.com>,
        syzkaller <syzkaller@googlegroups.com>
From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Fri, 6 Jan 2017 14:37:38 +0100
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On 06/01/2017 10:59, Wanpeng Li wrote:
> 2016-12-27 21:57 GMT+08:00 Dmitry Vyukov <dvyukov@google.com>:
>> Hello,
>>
>> The following program triggers use-after-free in complete_emulated_mmio:
>> https://gist.githubusercontent.com/dvyukov/79c7ee10f568b0d5c33788534bb6edc9/raw/2c2d4ce0fe86398ed81e65281e8c215c7c3632fb/gistfile1.txt
>>
>> BUG: KASAN: use-after-free in complete_emulated_mmio+0x8dd/0xb70
>> arch/x86/kvm/x86.c:7052 at addr ffff880069f1ed48
>> Read of size 8 by task syz-executor/31542
>> CPU: 3 PID: 31542 Comm: syz-executor Not tainted 4.9.0+ #105
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>>  check_memory_region+0x139/0x190 mm/kasan/kasan.c:322
>>  memcpy+0x23/0x50 mm/kasan/kasan.c:357
>>  complete_emulated_mmio+0x8dd/0xb70 arch/x86/kvm/x86.c:7052
>>  kvm_arch_vcpu_ioctl_run+0x308d/0x45f0 arch/x86/kvm/x86.c:7090
>>  kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
>>  vfs_ioctl fs/ioctl.c:43 [inline]
>>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
>>  SYSC_ioctl fs/ioctl.c:698 [inline]
>>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>>  entry_SYSCALL_64_fastpath+0x1f/0xc2
>> RIP: 0033:0x4421e9
>> RSP: 002b:00007f320dc67b58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
>> RAX: ffffffffffffffda RBX: 0000000000000018 RCX: 00000000004421e9
>> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000018
>> RBP: 00000000006dbb20 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000700000
>> R13: 00007f320de671c8 R14: 00007f320de69000 R15: 0000000000000000
>> Object at ffff880069f183c0, in cache kmalloc-16384 size: 16384
>> Allocated:
>> PID = 31567
>> [<ffffffff8123eb36>] save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
>> [<ffffffff81943353>] save_stack+0x43/0xd0 mm/kasan/kasan.c:502
>> [<ffffffff8194361a>] set_track mm/kasan/kasan.c:514 [inline]
>> [<ffffffff8194361a>] kasan_kmalloc+0xaa/0xd0 mm/kasan/kasan.c:605
>> [<ffffffff8193fa7c>] kmem_cache_alloc_trace+0xec/0x640 mm/slab.c:3629
>> [<ffffffff810724ce>] kvm_arch_alloc_vm include/linux/slab.h:490 [inline]
>> [<ffffffff810724ce>] kvm_create_vm
>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:613 [inline]
>> [<ffffffff810724ce>] kvm_dev_ioctl_create_vm
>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:3174 [inline]
>> [<ffffffff810724ce>] kvm_dev_ioctl+0x1be/0x11b0
>> arch/x86/kvm/../../../virt/kvm/kvm_main.c:3218
>> [<ffffffff819af76f>] vfs_ioctl fs/ioctl.c:43 [inline]
>> [<ffffffff819af76f>] do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
>> [<ffffffff819b0dbf>] SYSC_ioctl fs/ioctl.c:698 [inline]
>> [<ffffffff819b0dbf>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>> [<ffffffff83fd1d81>] entry_SYSCALL_64_fastpath+0x1f/0xc2
>> Memory state around the buggy address:
>>  ffff880069f1ec00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>  ffff880069f1ec80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>> ffff880069f1ed00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>                                               ^
>>  ffff880069f1ed80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>  ffff880069f1ee00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>> ==================================================================
>>
>>
>> On commit e93b1cc8a8965da137ffea0b88e5f62fa1d2a9e6 (Dec 19).
>>
>>
>> I've also printed some values when the bug happens:
>>
>> pr_err("vcpu=%p, mmio_fragments=%p frag=%p frag=%d/%d len=%d gpa=%p write=%d\n",
>> vcpu, vcpu->mmio_fragments, frag, vcpu->mmio_cur_fragment,
>> vcpu->mmio_nr_fragments, frag->len, (void*)frag->gpa,
>> vcpu->mmio_is_write);
>>
>> [   26.765898] vcpu=ffff880068590100, mmio_fragments=ffff880068590338
>> frag=ffff880068590338 frag=0/1 len=152 gpa=0000000000001008 write=1
> 
> 
> test-2892  [006] ....   118.284172: complete_emulated_mmio: vcpu =
> ffff9beefb288000, mmio_fragments = ffff9beefb2881b0, frag =
> ffff9beefb2881b0, frag = 0/1, len = 160, gpa = 0000000000001000, write
> = 1
> test-2897  [003] ....   118.284196: complete_emulated_mmio: vcpu =
> ffff9beef69a0000, mmio_fragments = ffff9beef69a01b0, frag =
> ffff9beef69a01b0, frag = 0/1, len = 160, gpa = 0000000000001000, write
> = 1
> 
> Actually the mmio will be splitted to 8 byte piece and returns to
> qemu(if it's not emulated by kvm) to be emulated one by one, however,
> we can observe that there is no subsequent handle to the left pieces,
> I guess the VM is almost destroyed immediately in the testcase, right?

Hi Wanpeng, yeah, there are only two KVM_RUN.  This bug was discussed
off list and we'll send a patch soon.

Paolo