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

[Stephan Mueller]

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)
>
>Bye,
>Hannes


Ciao
Stephan
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