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

[Stephan Mueller]

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?
>
>Bye,
>Hannes


Ciao
Stephan
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