crypto: memzero_explicit - make sure to clear out sensitive data

Recently, in commit 13aa93c70e71 ("random: add and use memzero_explicit()
for clearing data"), we have found that GCC may optimize some memset()
cases away when it detects a stack variable is not being used anymore
and going out of scope. This can happen, for example, in cases when we
are clearing out sensitive information such as keying material or any
e.g. intermediate results from crypto computations, etc.

With the help of Coccinelle, we can figure out and fix such occurences
in the crypto subsytem as well. Julia Lawall provided the following
Coccinelle program:

  @@
  type T;
  identifier x;
  @@

  T x;
  ... when exists
      when any
  -memset
  +memzero_explicit
     (&x,
  -0,
     ...)
  ... when != x
      when strict

  @@
  type T;
  identifier x;
  @@

  T x[...];
  ... when exists
      when any
  -memset
  +memzero_explicit
     (x,
  -0,
     ...)
  ... when != x
      when strict

Therefore, make use of the drop-in replacement memzero_explicit() for
exactly such cases instead of using memset().

Signed-off-by: Daniel Borkmann <[email protected]>
Cc: Julia Lawall <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Theodore Ts'o <[email protected]>
Cc: Hannes Frederic Sowa <[email protected]>
Acked-by: Hannes Frederic Sowa <[email protected]>
Acked-by: Herbert Xu <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>

crypto/vmac.c

@@ -613,7 +613,7 @@ static int vmac_final(struct shash_desc *pdesc, u8 *out)
 	}
 	mac = vmac(ctx->partial, ctx->partial_size, nonce, NULL, ctx);
 	memcpy(out, &mac, sizeof(vmac_t));
-	memset(&mac, 0, sizeof(vmac_t));
+	memzero_explicit(&mac, sizeof(vmac_t));
 	memset(&ctx->__vmac_ctx, 0, sizeof(struct vmac_ctx));
 	ctx->partial_size = 0;
 	return 0;