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__. Thanks a lot mancha, could you send a patch? Best, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
