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From: Vlastimil Babka <vbabka@suse.cz>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Dmitry Vyukov <dvyukov@google.com>,
	syzbot 
	<bot+6a5269ce759a7bb12754ed9622076dc93f65a1f6@syzkaller.appspotmail.com>,
	Jan Beulich <JBeulich@suse.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	ldufour@linux.vnet.ibm.com, LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@redhat.com>,
	syzkaller-bugs@googlegroups.com,
	Thomas Gleixner <tglx@linutronix.de>,
	the arch/x86 maintainers <x86@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>, Hugh Dickins <hughd@google.com>,
	David Rientjes <rientjes@google.com>,
	linux-mm@kvack.org, Andrea Arcangeli <aarcange@redhat.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Thorsten Leemhuis <regressions@leemhuis.info>
Subject: Re: KASAN: use-after-free Read in __do_page_fault
Date: Tue, 31 Oct 2017 15:28:26 +0100	[thread overview]
Message-ID: <fbf1e43d-1f73-09c1-1837-3600bcedd5d2@suse.cz> (raw)
In-Reply-To: <20171031141152.tzx47fy26pvx7xug@node.shutemov.name>

On 10/31/2017 03:11 PM, Kirill A. Shutemov wrote:
> 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?
 
Hmm that could indeed work, Dmitry can you try the patch below?
But it still seems rather fragile so I'd hope Andrea can do it more
robust, or at least make sure that we don't reintroduce this kind of
problem in the future (explicitly set vma to NULL with a comment?).

----8<----
diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index e2baeaa053a5..9bd16fc621db 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -1441,6 +1441,7 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 	 * the fault.  Since we never set FAULT_FLAG_RETRY_NOWAIT, if
 	 * we get VM_FAULT_RETRY back, the mmap_sem has been unlocked.
 	 */
+	pkey = vma_pkey(vma);
 	fault = handle_mm_fault(vma, address, flags);
 	major |= fault & VM_FAULT_MAJOR;
 
@@ -1467,7 +1468,6 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
 		return;
 	}
 
-	pkey = vma_pkey(vma);
 	up_read(&mm->mmap_sem);
 	if (unlikely(fault & VM_FAULT_ERROR)) {
 		mm_fault_error(regs, error_code, address, &pkey, fault);

  reply	other threads:[~2017-10-31 14:28 UTC|newest]

Thread overview: 17+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-10-30 19:12 syzbot
2017-10-30 19:15 ` Dmitry Vyukov
2017-10-31 12:00   ` Vlastimil Babka
2017-10-31 12:42     ` Dmitry Vyukov
2017-10-31 13:20       ` Vlastimil Babka
2017-10-31 13:57         ` Vlastimil Babka
2017-10-31 14:11           ` Kirill A. Shutemov
2017-10-31 14:28             ` Vlastimil Babka [this message]
2017-10-31 19:15               ` Andrea Arcangeli
2017-11-01  7:42                 ` Vlastimil Babka
2017-11-01 10:17                   ` Andrea Arcangeli
2017-11-01 12:14                     ` Vlastimil Babka
2017-10-31 15:37           ` Linus Torvalds
2017-10-31 19:13             ` Andrea Arcangeli
2017-11-01 15:26               ` Linus Torvalds
2017-11-02 19:36                 ` Andrea Arcangeli
2017-11-02 10:00           ` Laurent Dufour

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