Re: [BUG/PATCH] kernel RNG and its secrets

[Daniel Borkmann <daniel@iogearbox.net>]

On 03/18/2015 01:20 PM, Stephan Mueller wrote:
> Am Mittwoch, 18. März 2015, 13:19:07 schrieb Hannes Frederic Sowa:
>
> Hi Hannes,
>
>> On Wed, Mar 18, 2015, at 13:14, Stephan Mueller wrote:
>>> Am Mittwoch, 18. März 2015, 13:02:12 schrieb Hannes Frederic Sowa:
>>>
>>> Hi Hannes,
>>>
>>>> On Wed, Mar 18, 2015, at 12:09, Stephan Mueller wrote:
>>>>> Am Mittwoch, 18. März 2015, 11:56:43 schrieb Daniel Borkmann:
>>>>>> On 03/18/2015 11:50 AM, Hannes Frederic Sowa wrote:
>>>>>>> On Wed, Mar 18, 2015, at 10:53, mancha wrote:
>>>>>>>> Hi.
>>>>>>>>
>>>>>>>> The kernel RNG introduced memzero_explicit in d4c5efdb9777 to
>>>>>>>> protect
>>>>>>>>
>>>>>>>> memory cleansing against things like dead store optimization:
>>>>>>>>      void memzero_explicit(void *s, size_t count)
>>>>>>>>      {
>>>>>>>>
>>>>>>>>              memset(s, 0, count);
>>>>>>>>              OPTIMIZER_HIDE_VAR(s);
>>>>>>>>
>>>>>>>>      }
>>>>>>>>
>>>>>>>> OPTIMIZER_HIDE_VAR, introduced in fe8c8a126806 to protect
>>>>>>>> crypto_memneq>>
>>>>>>>>
>>>>>>>> against timing analysis, is defined when using gcc as:
>>>>>>>>      #define OPTIMIZER_HIDE_VAR(var) __asm__ ("" : "=r" (var) :
>>>>>>>>      "0"
>>>>>>>>      (var))
>>>>>>>>
>>>>>>>> My tests with gcc 4.8.2 on x86 find it insufficient to prevent
>>>>>>>> gcc
>>>>>>>> from optimizing out memset (i.e. secrets remain in memory).
>>>>>>>>
>>>>>>>> Two things that do work:
>>>>>>>>      __asm__ __volatile__ ("" : "=r" (var) : "0" (var))
>>>>>>>
>>>>>>> You are correct, volatile signature should be added to
>>>>>>> OPTIMIZER_HIDE_VAR. Because we use an output variable "=r", gcc
>>>>>>> is
>>>>>>> allowed to check if it is needed and may remove the asm
>>>>>>> statement.
>>>>>>> Another option would be to just use var as an input variable -
>>>>>>> asm
>>>>>>> blocks without output variables are always considered being
>>>>>>> volatile
>>>>>>> by gcc.
>>>>>>>
>>>>>>> Can you send a patch?
>>>>>>>
>>>>>>> I don't think it is security critical, as Daniel pointed out,
>>>>>>> the
>>>>>>> call
>>>>>>> will happen because the function is an external call to the
>>>>>>> crypto
>>>>>>> functions, thus the compiler has to flush memory on return.
>>>>>>
>>>>>> Just had a look.
>>>>>>
>>>>>> $ gdb vmlinux
>>>>>> (gdb) disassemble memzero_explicit
>>>>>>
>>>>>> Dump of assembler code for function memzero_explicit:
>>>>>>     0xffffffff813a18b0 <+0>:	push   %rbp
>>>>>>     0xffffffff813a18b1 <+1>:	mov    %rsi,%rdx
>>>>>>     0xffffffff813a18b4 <+4>:	xor    %esi,%esi
>>>>>>     0xffffffff813a18b6 <+6>:	mov    %rsp,%rbp
>>>>>>     0xffffffff813a18b9 <+9>:	callq  0xffffffff813a7120
>>>
>>> <memset>
>>>
>>>>>>     0xffffffff813a18be <+14>:	pop    %rbp
>>>>>>     0xffffffff813a18bf <+15>:	retq
>>>>>>
>>>>>> End of assembler dump.
>>>>>>
>>>>>> (gdb) disassemble extract_entropy
>>>>>> [...]
>>>>>>
>>>>>>     0xffffffff814a5000 <+304>:	sub    %r15,%rbx
>>>>>>     0xffffffff814a5003 <+307>:	jne    0xffffffff814a4f80
>>>>>>
>>>>>> <extract_entropy+176> 0xffffffff814a5009 <+313>:	mov    %r12,%rdi
>>>>>>
>>>>>>     0xffffffff814a500c <+316>:	mov    $0xa,%esi
>>>>>>     0xffffffff814a5011 <+321>:	callq  0xffffffff813a18b0
>>>>>>
>>>>>> <memzero_explicit> 0xffffffff814a5016 <+326>:	mov
>>>>>> -0x48(%rbp),%rax
>>>>>> [...]
>>>>>>
>>>>>> I would be fine with __volatile__.
>>>>>
>>>>> Are we sure that simply adding a __volatile__ works in any case? I
>>>>> just did a test with a simple user space app:
>>>>>
>>>>> static inline void memset_secure(void *s, int c, size_t n)
>>>>> {
>>>>>
>>>>>          memset(s, c, n);
>>>>>          //__asm__ __volatile__("": : :"memory");
>>>>>          __asm__ __volatile__("" : "=r" (s) : "0" (s));
>>>>>
>>>>> }
>>>>
>>>> Good point, thanks!
>>>>
>>>> Of course an input or output of s does not force the memory pointed
>>>> to
>>>> by s being flushed.
>>>>
>>>>
>>>> My proposal would be to add a
>>>>
>>>> #define OPTIMIZER_HIDE_MEM(ptr, len) __asm__ __volatile__ ("" : :
>>>> "m"(
>>>> ({ struct { u8 b[len]; } *p = (void *)ptr ; *p; }) )
>>>>
>>>> and use this in the code function.
>>>>
>>>> This is documented in gcc manual 6.43.2.5.
>>>
>>> That one adds the zeroization instructuctions. But now there are much
>>> more than with the barrier.
>>>
>>>    400469:       48 c7 04 24 00 00 00    movq   $0x0,(%rsp)
>>>    400470:       00
>>>    400471:       48 c7 44 24 08 00 00    movq   $0x0,0x8(%rsp)
>>>    400478:       00 00
>>>    40047a:       c7 44 24 10 00 00 00    movl   $0x0,0x10(%rsp)
>>>    400481:       00
>>>    400482:       48 c7 44 24 20 00 00    movq   $0x0,0x20(%rsp)
>>>    400489:       00 00
>>>    40048b:       48 c7 44 24 28 00 00    movq   $0x0,0x28(%rsp)
>>>    400492:       00 00
>>>    400494:       c7 44 24 30 00 00 00    movl   $0x0,0x30(%rsp)
>>>    40049b:       00
>>>
>>> Any ideas?
>>
>> Hmm, correct definition of u8?
>
> I use unsigned char
>>
>> Which version of gcc do you use? I can't see any difference if I
>> compile your example at -O2.
>
> gcc-Version 4.9.2 20150212 (Red Hat 4.9.2-6) (GCC)

I can see the same with the gcc version I previously posted. So
it clears the 20 bytes from your example (movq, movq, movl) at
two locations, presumably buf[] and b[].

Best,
Daniel