lib: memzero_explicit: use barrier instead of OPTIMIZER_HIDE_VAR
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: d4c5efdb ("random: add and use memzero_explicit() for clearing data") Cc: Theodore Ts'o <tytso@mit.edu> Signed-off-by:mancha security <mancha1@zoho.com> Signed-off-by:
Daniel Borkmann <daniel@iogearbox.net> Acked-by:
Hannes Frederic Sowa <hannes@stressinduktion.org> Acked-by:
Stephan Mueller <smueller@chronox.de> Signed-off-by:
Herbert Xu <herbert@gondor.apana.org.au>
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