Re: KASAN: use-after-free Read in __do_page_fault

[Laurent Dufour <ldufour@linux.vnet.ibm.com>]

Hi Vlastimil,

Sorry for the late answer I got a few day off.

On 31/10/2017 14:57, Vlastimil Babka wrote:
> +CC Andrea, Thorsten, Linus
> 
> On 10/31/2017 02:20 PM, Vlastimil Babka wrote:
>> On 10/31/2017 01:42 PM, Dmitry Vyukov wrote:
>>>> My vm_area_struct is 192 bytes, could be your layout is different due to
>>>> .config. At offset 80 I have vma->vm_flags. That is checked by
>>>> __do_page_fault(), but only after vma->vm_start (offset 0). Of course,
>>>> reordering is possible.
>>>
>>>
>>> It seems that compiler over-optimizes things and messes debug info.
>>> I just re-reproduced this on upstream
>>> 15f859ae5c43c7f0a064ed92d33f7a5bc5de6de0 and got the same report:
>>>
>>> ==================================================================
>>> BUG: KASAN: use-after-free in arch_local_irq_enable
>>> arch/x86/include/asm/paravirt.h:787 [inline]
>>> BUG: KASAN: use-after-free in __do_page_fault+0xc03/0xd60
>>> arch/x86/mm/fault.c:1357
>>> Read of size 8 at addr ffff880064d19aa0 by task syz-executor/8001
>>>
>>> CPU: 0 PID: 8001 Comm: syz-executor Not tainted 4.14.0-rc6+ #12
>>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>>> Call Trace:
>>>  __dump_stack lib/dump_stack.c:16 [inline]
>>>  dump_stack+0x194/0x257 lib/dump_stack.c:52
>>>  print_address_description+0x73/0x250 mm/kasan/report.c:252
>>>  kasan_report_error mm/kasan/report.c:351 [inline]
>>>  kasan_report+0x25b/0x340 mm/kasan/report.c:409
>>>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:430
>>>  arch_local_irq_enable arch/x86/include/asm/paravirt.h:787 [inline]
>>>  __do_page_fault+0xc03/0xd60 arch/x86/mm/fault.c:1357
>>>  do_page_fault+0xee/0x720 arch/x86/mm/fault.c:1520
>>>  do_async_page_fault+0x82/0x110 arch/x86/kernel/kvm.c:273
>>>  async_page_fault+0x22/0x30 arch/x86/entry/entry_64.S:1069
>>> RIP: 0033:0x441bd0
>>> RSP: 002b:00007f2ed8229798 EFLAGS: 00010202
>>> RAX: 00007f2ed82297c0 RBX: 0000000000000000 RCX: 000000000000000e
>>> RDX: 0000000000000400 RSI: 0000000020012fe0 RDI: 00007f2ed82297c0
>>> RBP: 0000000000748020 R08: 0000000000000400 R09: 0000000000000000
>>> R10: 0000000020012fee R11: 0000000000000246 R12: 00000000ffffffff
>>> R13: 0000000000008430 R14: 00000000006ec4d0 R15: 00007f2ed822a700
>>>
>>> Allocated by task 8001:
>>>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
>>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>>>  set_track mm/kasan/kasan.c:459 [inline]
>>>  kasan_kmalloc+0xad/0xe0 mm/kasan/kasan.c:551
>>>  kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:489
>>>  kmem_cache_alloc+0x12e/0x760 mm/slab.c:3561
>>>  kmem_cache_zalloc include/linux/slab.h:656 [inline]
>>>  mmap_region+0x7ee/0x15a0 mm/mmap.c:1658
>>>  do_mmap+0x69b/0xd40 mm/mmap.c:1468
>>>  do_mmap_pgoff include/linux/mm.h:2150 [inline]
>>>  vm_mmap_pgoff+0x1de/0x280 mm/util.c:333
>>>  SYSC_mmap_pgoff mm/mmap.c:1518 [inline]
>>>  SyS_mmap_pgoff+0x23b/0x5f0 mm/mmap.c:1476
>>>  SYSC_mmap arch/x86/kernel/sys_x86_64.c:99 [inline]
>>>  SyS_mmap+0x16/0x20 arch/x86/kernel/sys_x86_64.c:90
>>>  entry_SYSCALL_64_fastpath+0x1f/0xbe
>>>
>>> Freed by task 8007:
>>>  save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
>>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:447
>>>  set_track mm/kasan/kasan.c:459 [inline]
>>>  kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
>>>  __cache_free mm/slab.c:3503 [inline]
>>>  kmem_cache_free+0x77/0x280 mm/slab.c:3763
>>>  remove_vma+0x162/0x1b0 mm/mmap.c:176
>>>  remove_vma_list mm/mmap.c:2475 [inline]
>>>  do_munmap+0x82a/0xdf0 mm/mmap.c:2714
>>>  mmap_region+0x59e/0x15a0 mm/mmap.c:1631
>>>  do_mmap+0x69b/0xd40 mm/mmap.c:1468
>>>  do_mmap_pgoff include/linux/mm.h:2150 [inline]
>>>  vm_mmap_pgoff+0x1de/0x280 mm/util.c:333
>>>  SYSC_mmap_pgoff mm/mmap.c:1518 [inline]
>>>  SyS_mmap_pgoff+0x23b/0x5f0 mm/mmap.c:1476
>>>  SYSC_mmap arch/x86/kernel/sys_x86_64.c:99 [inline]
>>>  SyS_mmap+0x16/0x20 arch/x86/kernel/sys_x86_64.c:90
>>>  entry_SYSCALL_64_fastpath+0x1f/0xbe
>>>
>>> The buggy address belongs to the object at ffff880064d19a50
>>>  which belongs to the cache vm_area_struct of size 200
>>> The buggy address is located 80 bytes inside of
>>>  200-byte region [ffff880064d19a50, ffff880064d19b18)
>>> The buggy address belongs to the page:
>>> page:ffffea0001934640 count:1 mapcount:0 mapping:ffff880064d19000 index:0x0
>>> flags: 0x100000000000100(slab)
>>> raw: 0100000000000100 ffff880064d19000 0000000000000000 000000010000000f
>>> raw: ffffea00018a3a60 ffffea0001940be0 ffff88006c5f79c0 0000000000000000
>>> page dumped because: kasan: bad access detected
>>>
>>> Memory state around the buggy address:
>>>  ffff880064d19980: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>  ffff880064d19a00: fb fb fc fc fc fc fc fc fc fc fb fb fb fb fb fb
>>>> ffff880064d19a80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>                                ^
>>>  ffff880064d19b00: fb fb fb fc fc fc fc fc fc fc fc fb fb fb fb fb
>>>  ffff880064d19b80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>> ==================================================================
>>>
>>>
>>> Here is disasm of the function:
>>> https://gist.githubusercontent.com/dvyukov/5a56c66ce605168c951a321d94df6e3a/raw/538d4ce72ceb5631dfcc866ccde46c74543de1cf/gistfile1.txt
>>>
>>> Seems to be vma->vm_flags at offset 80.
>>
>> You can see it from the disasm? I can't make much of it, unfortunately,
>> the added kasan calls obscure it a lot for me. But I suspect it might be
>> the vma_pkey() thing which reads from vma->vm_flags. What happens when
>> CONFIG_X86_INTEL_MEMORY_PROTECTION_KEYS is disabled? (or is it already?)
> 
> OK, so I opened the google groups link in the report's signature and
> looked at the attached config there, which says protkeys are enabled.
> Also looked at the repro.txt attachment:
> #{Threaded:true Collide:true Repeat:true Procs:8 Sandbox:none Fault:false FaultCall:-1 FaultNth:0 EnableTun:true UseTmpDir:true HandleSegv:true WaitRepeat:true Debug:false Repro:false}
> mmap(&(0x7f0000000000/0xfff000)=nil, 0xfff000, 0x3, 0x32, 0xffffffffffffffff, 0x0)
> mmap(&(0x7f0000011000/0x3000)=nil, 0x3000, 0x1, 0x32, 0xffffffffffffffff, 0x0)
> r0 = userfaultfd(0x0)
> ioctl$UFFDIO_API(r0, 0xc018aa3f, &(0x7f0000002000-0x18)={0xaa, 0x0, 0x0})
> ioctl$UFFDIO_REGISTER(r0, 0xc020aa00, &(0x7f0000019000)={{&(0x7f0000012000/0x2000)=nil, 0x2000}, 0x1, 0x0})
> r1 = gettid()
> syz_open_dev$evdev(&(0x7f0000013000-0x12)="2f6465762f696e7075742f6576656e742300", 0x0, 0x0)
> tkill(r1, 0x7)
> 
> The userfaultfd() caught my attention so I checked handle_userfault()
> which seems to do up_read(&mm->mmap_sem); and in some cases later
> followed by down_read(&mm->mmap_sem); return VM_FAULT_NOPAGE.
> However, __do_page_fault() only expects that mmap_sem to be released
> when handle_mm_fault() returns with VM_FAULT_RETRY. It doesn't expect it
> to be released and then acquired again, because then vma can be indeed
> gone. It seems vma hasn't been touched after that point until the
> vma_pkey() was added by commit a3c4fb7c9c2e ("x86/mm: Fix fault error
> path using unsafe vma pointer") in rc3. Which tried to fix a similar
> problem, but run into this corner case?
> 
> So I suspect a3c4fb7c9c2e is the culprit and thus a regression.

Yes that's my mistake.

My patch was removing the use of vma once mmap_sem is released but it was
mainly done in the error path and I moved the read of the vma's pkey before
releasing the mmap_sem, but in the no-error path also, leading to the use
after free you seen.

As suggested and done later in this thread, reading the vma's key value
before calling handle_mm_fault() will solve this issue. This is safe since
the vma's pkey can't be changed once the mmap_sem is held.

Thanks,
Laurent.

> 
>> Also did you try the vmacache shortcut test suggested in my previous mail?
>>
>>> I think the size of 200 reported by slab is OK as it can do some rounding.
>>> Everything points to a vma object.
>>>
>>>
>>>>>> The buggy address belongs to the page:
>>>>>> page:ffffea00072ff4c0 count:1 mapcount:0 mapping:ffff8801cbfd3040 index:0x0
>>>>>> flags: 0x200000000000100(slab)
>>>>>> raw: 0200000000000100 ffff8801cbfd3040 0000000000000000 000000010000000f
>>>>>> raw: ffffea000730c7a0 ffffea00072ff7a0 ffff8801dae069c0 0000000000000000
>>>>>> page dumped because: kasan: bad access detected
>>>>>>
>>>>>> Memory state around the buggy address:
>>>>>>  ffff8801cbfd2f80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>>>>>>  ffff8801cbfd3000: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
>>>>>>>
>>>>>>> ffff8801cbfd3080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>>>>
>>>>>>                          ^
>>>>>>  ffff8801cbfd3100: fb fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb
>>>>>>  ffff8801cbfd3180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>>>> ==================================================================
>>>>>
>>>>>
>>>>> I guess this is more related to mm rather than x86, so +mm maintainers.
>>>>> This continues to happen, in particular on upstream
>>>>> 781402340475144bb360e32bb7437fa4b84cadc3 (Oct 28).
>>>>>
>>>>>
>>>>>> ---
>>>>>> This bug is generated by a dumb bot. It may contain errors.
>>>>>> See https://goo.gl/tpsmEJ for details.
>>>>>> Direct all questions to syzkaller@googlegroups.com.
>>>>>>
>>>>>> syzbot will keep track of this bug report.
>>>>>> Once a fix for this bug is committed, please reply to this email with:
>>>>>> #syz fix: exact-commit-title
>>>>>> To mark this as a duplicate of another syzbot report, please reply with:
>>>>>> #syz dup: exact-subject-of-another-report
>>>>>> If it's a one-off invalid bug report, please reply with:
>>>>>> #syz invalid
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>>>>>
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>>
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>