Date:   Tue, 11 Apr 2017 16:36:30 +0100
From:   Marc Zyngier <marc.zyngier@....com>
To:     Andrey Konovalov <andreyknvl@...gle.com>
Cc:     Suzuki K Poulose <Suzuki.Poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Suraj Jitindar Singh <sjitindarsingh@...il.com>,
        Markus Elfring <elfring@...rs.sourceforge.net>,
        Lorenzo Stoakes <lstoakes@...il.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Kostya Serebryany <kcc@...gle.com>,
        syzkaller <syzkaller@...glegroups.com>
Subject: Re: kvm/arm64: use-after-free in kvm_vm_ioctl/vmacache_update

On 11/04/17 16:26, Andrey Konovalov wrote:
> On Tue, Mar 14, 2017 at 1:26 PM, Marc Zyngier <marc.zyngier@....com> wrote:
>> On 14/03/17 11:03, Suzuki K Poulose wrote:
>>> On 13/03/17 09:58, Marc Zyngier wrote:
>>>> On 10/03/17 18:37, Suzuki K Poulose wrote:
>>>>> On 10/03/17 15:50, Andrey Konovalov wrote:
>>>>>> On Fri, Mar 10, 2017 at 2:38 PM, Andrey Konovalov <andreyknvl@...gle.com> wrote:
>>>>>>> Hi,
>>>>>>>
>>>>>>> I've got the following error report while fuzzing the kernel with syzkaller.
>>>>>>>
>>>>>>> On linux-next commit 56b8bad5e066c23e8fa273ef5fba50bd3da2ace8 (Mar 8).
>>>>>>>
>>>>>>> Unfortunately I can't reproduce it.
>>>>>>>
>>>>>>> ==================================================================
>>>>>>> BUG: KASAN: use-after-free in vmacache_update+0x114/0x118 mm/vmacache.c:63
>>>>>>> Read of size 8 at addr ffff80003b9a2040 by task syz-executor/26615
>>>>>>>
>>>>>>> CPU: 1 PID: 26615 Comm: syz-executor Not tainted
>>>>>>> 4.11.0-rc1-next-20170308-xc2-dirty #3
>>>>>>> Hardware name: Hardkernel ODROID-C2 (DT)
>>>>>>> Call trace:
>>>>>>> [<ffff20000808fbb0>] dump_backtrace+0x0/0x440 arch/arm64/kernel/traps.c:505
>>>>>>> [<ffff200008090010>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:228
>>>>>>> [<ffff2000088e9578>] __dump_stack lib/dump_stack.c:16 [inline]
>>>>>>> [<ffff2000088e9578>] dump_stack+0x110/0x168 lib/dump_stack.c:52
>>>>>>> [<ffff200008414018>] print_address_description+0x60/0x248 mm/kasan/report.c:250
>>>>>>> [<ffff2000084142e8>] kasan_report_error+0xe8/0x250 mm/kasan/report.c:349
>>>>>>> [<ffff200008414564>] kasan_report mm/kasan/report.c:372 [inline]
>>>>>>> [<ffff200008414564>] __asan_report_load8_noabort+0x3c/0x48 mm/kasan/report.c:393
>>>>>>> [<ffff200008383f64>] vmacache_update+0x114/0x118 mm/vmacache.c:63
>>>>>>> [<ffff2000083a9000>] find_vma+0xf8/0x150 mm/mmap.c:2124
>>>>>>> [<ffff2000080dc19c>] kvm_arch_prepare_memory_region+0x2ac/0x488
>>>>>>> arch/arm64/kvm/../../../arch/arm/kvm/mmu.c:1817
>>>>>>> [<ffff2000080c2920>] __kvm_set_memory_region+0x3d8/0x12b8
>>>>>>> arch/arm64/kvm/../../../virt/kvm/kvm_main.c:1026
>>>>>>> [<ffff2000080c3838>] kvm_set_memory_region+0x38/0x58
>>>>>>> arch/arm64/kvm/../../../virt/kvm/kvm_main.c:1075
>>>>>>> [<ffff2000080c747c>] kvm_vm_ioctl_set_memory_region
>>>>>>> arch/arm64/kvm/../../../virt/kvm/kvm_main.c:1087 [inline]
>>>>>>> [<ffff2000080c747c>] kvm_vm_ioctl+0xb94/0x1308
>>>>>>> arch/arm64/kvm/../../../virt/kvm/kvm_main.c:2960
>>>>>>> [<ffff20000848f928>] vfs_ioctl fs/ioctl.c:45 [inline]
>>>>>>> [<ffff20000848f928>] do_vfs_ioctl+0x128/0xfc0 fs/ioctl.c:685
>>>>>>> [<ffff200008490868>] SYSC_ioctl fs/ioctl.c:700 [inline]
>>>>>>> [<ffff200008490868>] SyS_ioctl+0xa8/0xb8 fs/ioctl.c:691
>>>>>>> [<ffff200008083f70>] el0_svc_naked+0x24/0x28
>>>>>>>
>>>>>>> Allocated by task 26657:
>>>>>>>  save_stack_trace_tsk+0x0/0x330 arch/arm64/kernel/stacktrace.c:133
>>>>>>>  save_stack_trace+0x20/0x30 arch/arm64/kernel/stacktrace.c:216
>>>>>>>  save_stack mm/kasan/kasan.c:515 [inline]
>>>>>>>  set_track mm/kasan/kasan.c:527 [inline]
>>>>>>>  kasan_kmalloc+0xd4/0x180 mm/kasan/kasan.c:619
>>>>>>>  kasan_slab_alloc+0x14/0x20 mm/kasan/kasan.c:557
>>>>>>>  slab_post_alloc_hook mm/slab.h:456 [inline]
>>>>>>>  slab_alloc_node mm/slub.c:2718 [inline]
>>>>>>>  slab_alloc mm/slub.c:2726 [inline]
>>>>>>>  kmem_cache_alloc+0x144/0x230 mm/slub.c:2731
>>>>>>>  __split_vma+0x118/0x608 mm/mmap.c:2515
>>>>>>>  do_munmap+0x194/0x9b0 mm/mmap.c:2636
>>>>>>> Freed by task 26657:
>>>>>>>  save_stack_trace_tsk+0x0/0x330 arch/arm64/kernel/stacktrace.c:133
>>>>>>>  save_stack_trace+0x20/0x30 arch/arm64/kernel/stacktrace.c:216
>>>>>>>  save_stack mm/kasan/kasan.c:515 [inline]
>>>>>>>  set_track mm/kasan/kasan.c:527 [inline]
>>>>>>>  kasan_slab_free+0x84/0x198 mm/kasan/kasan.c:592
>>>>>>>  slab_free_hook mm/slub.c:1357 [inline]
>>>>>>>  slab_free_freelist_hook mm/slub.c:1379 [inline]
>>>>>>>  slab_free mm/slub.c:2961 [inline]
>>>>>>>  kmem_cache_free+0x80/0x258 mm/slub.c:2983
>>>>>>>  __vma_adjust+0x6b0/0xf mm/mmap.c:890]  el0_svc_naked+0x24/0x28
>>>>>>>
>>>>>>> The buggy address belongs to the object at ffff80003b9a2000
>>>>>>>  which belongs to the cache vm_area_struct(647:session-6.scope) of size 184
>>>>>>> The buggy address is located 64 bytes inside of
>>>>>>>  184-byte region [ffff80003b9a2000, ffff80003b9a20b8)
>>>>>>> The buggy address belongs to the page:
>>>>>>> page:ffff7e0000ee6880 count:1 mapcount:0 mapping:          (null) index:0x0
>>>>>>> flags: 0xfffc00000000100(slab)
>>>>>>> raw: 0fffc00000000100 0000000000000000 0000000000000000 0000000180100010
>>>>>>> raw: 0000000000000000 0000000c00000001 ffff80005a5cc600 ffff80005ac99980
>>>>>>> page dumped because: kasan: bad access detected
>>>>>>> page->mem_cgroup:ffff80005ac99980
>>>>>>>
>>>>>>> Memory state around the buggy address:
>>>>>>>  ffff80003b9a1f00: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>>>>>>  ffff80003b9a1f80: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
>>>>>>>> ffff80003b9a2000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>>>>>                                            ^
>>>>>>>  ffff80003b9a2080: fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc fb
>>>>>>>  ffff80003b9a2100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>>>>> ==================================================================
>>>>>>
>>>>>> Another one that looks related and doesn't have parts of stack traces missing:
>>>>>>
>>>>>> ==================================================================
>>>>>> BUG: KASAN: use-after-free in find_vma+0x140/0x150 mm/mmap.c:2114
>>>>>> Read of size 8 at addr ffff800031a03e90 by task syz-executor/4360
>>>>>>
>>>>>> CPU: 2 PID: 4360 Comm: syz-executor Not tainted
>>>>>> 4.11.0-rc1-next-20170308-xc2-dirty #3
>>>>>> Hardware name: Hardkernel ODROID-C2 (DT)
>>>>>> Call trace:
>>>>>> [<ffff20000808fbb0>] dump_backtrace+0x0/0x440 arch/arm64/kernel/traps.c:505
>>>>>> [<ffff200008090010>] show_stack+0x20/0x30 arch/arm64/kernel/traps.c:228
>>>>>> [<ffff2000088e9578>] __dump_stack lib/dump_stack.c:16 [inline]
>>>>>> [<ffff2000088e9578>] dump_stack+0x110/0x168 lib/dump_stack.c:52
>>>>>> [<ffff200008414018>] print_address_description+0x60/0x248 mm/kasan/report.c:250
>>>>>> [<ffff2000084142e8>] kasan_report_error+0xe8/0x250 mm/kasan/report.c:349
>>>>>> [<ffff200008414564>] kasan_report mm/kasan/report.c:372 [inline]
>>>>>> [<ffff200008414564>] __asan_report_load8_noabort+0x3c/0x48 mm/kasan/report.c:393
>>>>>> [<ffff2000083a9048>] find_vma+0x140/0x150 mm/mmap.c:2114
>>>>>> [<ffff2000080dc19c>] kvm_arch_prepare_memory_region+0x2ac/0x488
>>>>>> arch/arm64/kvm/../../../arch/arm/kvm/mmu.c:1817
>>>>>
>>>>> It looks like we don't take the mmap_sem before calling find_vma() in
>>>>> stage2_unmap_memslot() and in kvm_arch_prepare_memory_region(), which is causing
>>>>> the race, with probably the test trying to unmap ranges in between.
>>>>
>>>> That indeed seems like a possible failure mode. The annoying thing is
>>>> that we're not exactly in a position to take mmap_sem in
>>>> stage2_unmap_memslot, since we hold the kvm->mmu_lock spinlock. We may
>>>> have to hold mmap_sem while iterating over all the memslots.
>>>>
>>>> How about the following (very lightly tested):
>>>>
>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>> index 962616fd4ddd..2006a79d5912 100644
>>>> --- a/arch/arm/kvm/mmu.c
>>>> +++ b/arch/arm/kvm/mmu.c
>>>> @@ -803,6 +803,7 @@ void stage2_unmap_vm(struct kvm *kvm)
>>>>      int idx;
>>>>
>>>>      idx = srcu_read_lock(&kvm->srcu);
>>>> +    down_read(&current->mm->mmap_sem);
>>>>      spin_lock(&kvm->mmu_lock);
>>>>
>>>>      slots = kvm_memslots(kvm);
>>>> @@ -810,6 +811,7 @@ void stage2_unmap_vm(struct kvm *kvm)
>>>>              stage2_unmap_memslot(kvm, memslot);
>>>>
>>>>      spin_unlock(&kvm->mmu_lock);
>>>> +    up_read(&current->mm->mmap_sem);
>>>>      srcu_read_unlock(&kvm->srcu, idx);
>>>>  }
>>>>
>>>> @@ -1813,6 +1815,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>>>       *     |               memory region                |
>>>>       *     +--------------------------------------------+
>>>>       */
>>>> +    down_read(&current->mm->mmap_sem);
>>>>      do {
>>>>              struct vm_area_struct *vma = find_vma(current->mm, hva);
>>>>              hva_t vm_start, vm_end;
>>>
>>> I have added the following hunk :
>>>
>>> @@ -1844,8 +1847,10 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>>>                          pa += vm_start - vma->vm_start;
>>>
>>>                          /* IO region dirty page logging not allowed */
>>> -                       if (memslot->flags & KVM_MEM_LOG_DIRTY_PAGES)
>>> -                               return -EINVAL;
>>> +                       if (memslot->flags & KVM_MEM_LOG_DIRTY_PAGES) {
>>> +                               ret = -EINVAL;
>>> +                               goto out;
>>> +                       }
>>>
>>>                          ret = kvm_phys_addr_ioremap(kvm, gpa, pa,
>>>                                                      vm_end - vm_start,
>>>
>>
>> Ah, of course... Thanks for pointing that out, I'll fix it as I post a
>> proper patch.
>>
>> Thanks,
>>
>>         M.
>> --
>> Jazz is not dead. It just smells funny...
> 
> Hi,
> 
> I've got this report again on linux-next 4c3c5cd02318 (Apr 5).
> 
> It seems that it's still not fixed.
> 
> Thanks!
> 
> ==================================================================
> BUG: KASAN: use-after-free in vmacache_update+0x114/0x118 mm/vmacache.c:63
> Read of size 8 at addr ffff80004d53aae8 by task syz-executor/23095
> 
> CPU: 0 PID: 23095 Comm: syz-executor Not tainted
> 4.11.0-rc5-next-20170405-xc2-09030-g4c3c5cd02318-dirty #4

The fixes went into -rc6. Can you please give it a go?

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...

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