Re: [PATCH -crypto] lib: memzero_explicit: use barrier instead of OPTIMIZER_HIDE_VAR

[Hannes Frederic Sowa <hannes@stressinduktion.org>]

On Wed, Mar 18, 2015, at 18:47, Daniel Borkmann wrote:
> From: mancha security <mancha1@zoho.com>
> 
> OPTIMIZER_HIDE_VAR(), as defined when using gcc, is insufficient to
> ensure protection from dead store optimization.
> 
> For the random driver and crypto drivers, calls are emitted ...
> 
>   $ 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
>   [...]
>     0xffffffff814a5009 <+313>:      mov    %r12,%rdi
>     0xffffffff814a500c <+316>:      mov    $0xa,%esi
>     0xffffffff814a5011 <+321>:      callq  0xffffffff813a18b0
>     <memzero_explicit>
>     0xffffffff814a5016 <+326>:      mov    -0x48(%rbp),%rax
>   [...]
> 
> ... but in case in future we might use facilities such as LTO, then
> OPTIMIZER_HIDE_VAR() is not sufficient to protect gcc from a possible
> eviction of the memset(). We have to use a compiler barrier instead.
> 
> Minimal test example when we assume memzero_explicit() would *not* be
> a call, but would have been *inlined* instead:
> 
>   static inline void memzero_explicit(void *s, size_t count)
>   {
>     memset(s, 0, count);
>     <foo>
>   }
> 
>   int main(void)
>   {
>     char buff[20];
> 
>     snprintf(buff, sizeof(buff) - 1, "test");
>     printf("%s", buff);
> 
>     memzero_explicit(buff, sizeof(buff));
>     return 0;
>   }
> 
> With <foo> := OPTIMIZER_HIDE_VAR():
> 
>   (gdb) disassemble main
>   Dump of assembler code for function main:
>   [...]
>    0x0000000000400464 <+36>:       callq  0x400410 <printf@plt>
>    0x0000000000400469 <+41>:       xor    %eax,%eax
>    0x000000000040046b <+43>:       add    $0x28,%rsp
>    0x000000000040046f <+47>:       retq
>   End of assembler dump.
> 
> With <foo> := barrier():
> 
>   (gdb) disassemble main
>   Dump of assembler code for function main:
>   [...]
>    0x0000000000400464 <+36>:       callq  0x400410 <printf@plt>
>    0x0000000000400469 <+41>:       movq   $0x0,(%rsp)
>    0x0000000000400471 <+49>:       movq   $0x0,0x8(%rsp)
>    0x000000000040047a <+58>:       movl   $0x0,0x10(%rsp)
>    0x0000000000400482 <+66>:       xor    %eax,%eax
>    0x0000000000400484 <+68>:       add    $0x28,%rsp
>    0x0000000000400488 <+72>:       retq
>   End of assembler dump.
> 
> As can be seen, movq, movq, movl are being emitted inlined
> via memset().
> 
> Reference: http://thread.gmane.org/gmane.linux.kernel.cryptoapi/13764/
> Fixes: d4c5efdb9777 ("random: add and use memzero_explicit() for clearing
> data")
> Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
> Cc: Stephan Mueller <smueller@chronox.de>
> Cc: Theodore Ts'o <tytso@mit.edu>
> Signed-off-by: mancha security <mancha1@zoho.com>
> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> ---
>  Sending to Herbert as crypto/random are the main users.
>  Based against -crypto tree. Thanks!

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>

Still checking on how to realize the test. Thanks!