Patch "lib: memzero_explicit: use barrier instead of OPTIMIZER_HIDE_VAR" has been added to the 3.10-stable tree

This is a note to let you know that I've just added the patch titled

lib: memzero_explicit: use barrier instead of OPTIMIZER_HIDE_VAR

to the 3.10-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
lib-memzero_explicit-use-barrier-instead-of-optimizer_hide_var.patch
and it can be found in the queue-3.10 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: mancha security <[email protected]>
Date: Wed, 18 Mar 2015 18:47:25 +0100
Subject: lib: memzero_explicit: use barrier instead of OPTIMIZER_HIDE_VAR

From: mancha security <[email protected]>

commit 0b053c9518292705736329a8fe20ef4686ffc8e9 upstream.

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/

---
 lib/string.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -598,7 +598,7 @@ EXPORT_SYMBOL(memset);
 void memzero_explicit(void *s, size_t count)
 {
 	memset(s, 0, count);
-	OPTIMIZER_HIDE_VAR(s);
+	barrier();
 }
 EXPORT_SYMBOL(memzero_explicit);
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.10/lib-memzero_explicit-use-barrier-instead-of-optimizer_hide_var.patch