Re: [PATCH] mm: don't allow fault_around_bytes to be 0

[Andrey Ryabinin]

On 07/28/14 13:36, Kirill A. Shutemov wrote:
> On Mon, Jul 28, 2014 at 11:43:20AM +0400, Andrey Ryabinin wrote:
>> Sasha Levin triggered use-after-free when fuzzing using trinity and the KASAN
>> patchset:
>>
>> 	AddressSanitizer: use after free in do_read_fault.isra.40+0x3c2/0x510 at addr ffff88048a733110
>> 	page:ffffea001229ccc0 count:0 mapcount:0 mapping:          (null) index:0x0
>> 	page flags: 0xafffff80008000(tail)
>> 	page dumped because: kasan error
>> 	CPU: 6 PID: 9262 Comm: trinity-c104 Not tainted 3.16.0-rc6-next-20140723-sasha-00047-g289342b-dirty #929
>> 	 00000000000000fb 0000000000000000 ffffea001229ccc0 ffff88038ac0fb78
>> 	 ffffffffa5e40903 ffff88038ac0fc48 ffff88038ac0fc38 ffffffffa142acfc
>> 	 0000000000000001 ffff880509ff5aa8 ffff88038ac10038 ffff88038ac0fbb0
>> 	Call Trace:
>> 	dump_stack (lib/dump_stack.c:52)
>> 	kasan_report_error (mm/kasan/report.c:98 mm/kasan/report.c:166)
>> 	? debug_smp_processor_id (lib/smp_processor_id.c:57)
>> 	? preempt_count_sub (kernel/sched/core.c:2606)
>> 	? put_lock_stats.isra.13 (./arch/x86/include/asm/preempt.h:98 kernel/locking/lockdep.c:254)
>> 	? do_read_fault.isra.40 (mm/memory.c:2784 mm/memory.c:2849 mm/memory.c:2898)
>> 	__asan_load8 (mm/kasan/kasan.c:364)
>> 	? do_read_fault.isra.40 (mm/memory.c:2864 mm/memory.c:2898)
>> 	do_read_fault.isra.40 (mm/memory.c:2864 mm/memory.c:2898)
>> 	? _raw_spin_unlock (./arch/x86/include/asm/preempt.h:98 include/linux/spinlock_api_smp.h:152 kernel/locking/spinlock.c:183)
>> 	? __pte_alloc (mm/memory.c:598)
>> 	handle_mm_fault (mm/memory.c:3092 mm/memory.c:3225 mm/memory.c:3345 mm/memory.c:3374)
>> 	? pud_huge (./arch/x86/include/asm/paravirt.h:611 arch/x86/mm/hugetlbpage.c:76)
>> 	__get_user_pages (mm/gup.c:286 mm/gup.c:478)
>> 	__mlock_vma_pages_range (mm/mlock.c:262)
>> 	__mm_populate (mm/mlock.c:710)
>> 	SyS_remap_file_pages (mm/mmap.c:2653 mm/mmap.c:2593)
>> 	tracesys (arch/x86/kernel/entry_64.S:541)
>> 	Read of size 8 by thread T9262:
>> 	Memory state around the buggy address:
>> 	 ffff88048a732e80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a732f00: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a732f80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a733000: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a733080: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	>ffff88048a733100: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	                         ^
>> 	 ffff88048a733180: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a733200: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a733280: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a733300: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>> 	 ffff88048a733380: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>
>>
>> It looks like that pte pointer is invalid in do_fault_around().
>> This could happen if fault_around_bytes is set to 0.
>> fault_around_pages() and fault_around_mask() calls rounddown_pow_of_to(fault_around_bytes)
>> The result of rounddown_pow_of_to is undefined if parameter == 0
>> (in my environment it returns 0x8000000000000000).
> 
> Ouch. Good catch!
> 
> Although, I'm not convinced that it caused the issue. Sasha, did you touch the
> debugfs handle?
> 

I suppose trinity could change it, no? I've got the very same spew after setting fault_around_bytes to 0.

>> One way to fix this would be to return 0 from fault_around_pages() if fault_around_bytes == 0,
>> however this would add extra code on fault path.
>>
>> So let's just forbid to set fault_around_bytes to zero.
>> Fault around is not used if fault_around_pages() <= 1, so if anyone doesn't want to use
>> it, fault_around_bytes could be set to any value in range [1, 2*PAGE_SIZE - 1]
>> instead of 0.
> 
>>From user point of view, 0 is perfectly fine. What about untested patch
> below?
> 

In case if we are not going to get rid of debugfs interface I would better keep
faul_around_bytes always roundded down, like in following patch: