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

[Kirill A. Shutemov]

On Tue, Oct 31, 2017 at 02:57:58PM +0100, 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.

I wounder if we can move "pkey = vma_pkey(vma);" before handle_mm_fault()?
pkey can't change during page fault handing, can it?

-- 
 Kirill A. Shutemov