Re: [PATCH] arch/x86: fix out-of-bounds in get_wchan()

[Dmitry Vyukov]

On Mon, Sep 28, 2015 at 11:37 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Mon, Sep 28, 2015 at 11:00:39AM +0200, Dmitry Vyukov wrote:
>> get_wchan() checks that fp is within stack bounds,
>> but then dereferences fp+8. This can crash kernel
>> or leak sensitive information. Also the function
>> operates on a potentially running stack, but does
>> not use READ_ONCE. As the result it can check that
>> one value is within stack bounds, but then deref
>> another value.
>>
>> Fix the bounds check and use READ_ONCE for all
>> volatile data.
>>
>> The bug was discovered with KASAN.
>>
>> Signed-off-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> ---
>> FTR, here is the KASAN report:
>>
>> [  124.575597] ERROR: AddressSanitizer: heap-buffer-overflow on address ffff88002e280000
>> [  124.578633] Accessed by thread T10915:
>> [  124.581050]   #2 ffffffff810dd423 in __tsan_read8 ??:0
>> [  124.581893]   #3 ffffffff8107c093 in get_wchan ./arch/x86/kernel/process_64.c:444
>> [  124.582763]   #4 ffffffff81342108 in do_task_stat array.c:0
>> [  124.583634]   #5 ffffffff81342dcc in proc_tgid_stat ??:0
>> [  124.584548]   #6 ffffffff8133c984 in proc_single_show base.c:0
>> [  124.585461]   #7 ffffffff812d18cc in seq_read ./fs/seq_file.c:222
>> [  124.586313]   #8 ffffffff8129e503 in vfs_read ??:0
>> [  124.587137]   #9 ffffffff8129f800 in SyS_read ??:0
>> [  124.587827]   #10 ffffffff81929bf5 in sysenter_dispatch ./arch/x86/ia32/ia32entry.S:164
>> [  124.588738]
>> [  124.593434] Shadow bytes around the buggy address:
>> [  124.594270]   ffff88002e27fd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> [  124.595339]   ffff88002e27fe00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> [  124.596453]   ffff88002e27fe80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> [  124.597466]   ffff88002e27ff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> [  124.598501]   ffff88002e27ff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> [  124.599629] =>ffff88002e280000:[fa]fa fa fa fa fa fa fa fa fa 00 00 00 00 00 00
>> [  124.600873]   ffff88002e280080: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>> [  124.601892]   ffff88002e280100: 00 fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>> [  124.603037]   ffff88002e280180: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
>> [  124.604047]   ffff88002e280200: fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd fd
>> [  124.605054]   ffff88002e280280: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fa fa
>> [  124.605993] Shadow byte legend (one shadow byte represents 8 application bytes):
>> [  124.606958]   Addressable:   00
>> [  124.607483]   Partially addressable: 01 02 03 04 05 06 07
>> [  124.608219]   Heap redzone:  fa
>> [  124.608724]   Heap kmalloc redzone:  fb
>> [  124.609249]   Freed heap region: fd
>> [  124.609753]   Shadow gap:fe
>> [  124.610292] =========================================================================
>> ---
>>  arch/x86/kernel/process_64.c | 12 +++++++-----
>>  1 file changed, 7 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
>> index 71d7849..a1fce34 100644
>> --- a/arch/x86/kernel/process_64.c
>> +++ b/arch/x86/kernel/process_64.c
>> @@ -506,17 +506,19 @@ unsigned long get_wchan(struct task_struct *p)
>>       if (!p || p == current || p->state == TASK_RUNNING)
>>               return 0;
>>       stack = (unsigned long)task_stack_page(p);
>> -     if (p->thread.sp < stack || p->thread.sp >= stack+THREAD_SIZE)
>> +     /* The task can be already running at this point, so tread carefully. */
>> +     fp = READ_ONCE(p->thread.sp);
>> +     if (fp < stack || fp >= stack+THREAD_SIZE)
>>               return 0;
>> -     fp = *(u64 *)(p->thread.sp);
>> +     fp = READ_ONCE(*(u64 *)fp);
>
> Why isn't this:
>
>         fp = READ_ONCE(*(u64 *)p->thread.sp);
>
> like the original code did?


Original code did:

     if (p->thread.sp < stack || p->thread.sp >= stack+THREAD_SIZE)
           return 0;
     fp = *(u64 *)(p->thread.sp);

p->thread.sp can change concurrently.
So we could check that p->thread.sp is within stack bounds, but then
dereference another value (which is already outside of bounds).




> Actually, the original code looks fishy to me too - it did access live
> stack three times. And shouldn't we be accessing it only once?
>
> I.e.,
>
>         fp_st = READ_ONCE(p->thread.sp);
>         if (fp_st < stack || fp_st >= stack + THREAD_SIZE)
>                 return 0;
>         fp = *(u64 *)fp_st;
>
> Hmm?

That's what my patch does.


> Maybe I'm not completely clear on how the whole locking happens here
> because we do
>
>         if (!p || p == current || p->state == TASK_RUNNING)
>                 return 0;
>
> earlier but apparently we can become TASK_RUNNING after the check...
>
> Also, shouldn't this one have a CVE number assigned or so due to the
> leakage potential?
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