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

[Vlastimil Babka]

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?)

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@xxxxxxxxxxxxxxxxx
>>>>
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>>