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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