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

[Hannes Frederic Sowa]

On Wed, Mar 18, 2015, at 12:09, Stephan Mueller wrote:
> Am Mittwoch, 18. MÃrz 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.

Bye,
Hannes


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