[PATCH v4 3/6] crypto: lib/sha256 - Don't clear temporary variables

From: Arvind Sankar <nivedita@alum.mit.edu>
To: Herbert Xu <herbert@gondor.apana.org.au>,
	"David S. Miller" <davem@davemloft.net>,
	"linux-crypto@vger.kernel.org" <linux-crypto@vger.kernel.org>,
	Eric Biggers <ebiggers@kernel.org>,
	David Laight <David.Laight@aculab.com>
Cc: linux-kernel@vger.kernel.org, Eric Biggers <ebiggers@google.com>
Subject: [PATCH v4 3/6] crypto: lib/sha256 - Don't clear temporary variables
Date: Sun, 25 Oct 2020 10:31:16 -0400	[thread overview]
Message-ID: <20201025143119.1054168-4-nivedita@alum.mit.edu> (raw)
In-Reply-To: <20201025143119.1054168-1-nivedita@alum.mit.edu>

The assignments to clear a through h and t1/t2 are optimized out by the
compiler because they are unused after the assignments.

Clearing individual scalar variables is unlikely to be useful, as they
may have been assigned to registers, and even if stack spilling was
required, there may be compiler-generated temporaries that are
impossible to clear in any case.

So drop the clearing of a through h and t1/t2.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Reviewed-by: Eric Biggers <ebiggers@google.com>
---
 lib/crypto/sha256.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/lib/crypto/sha256.c b/lib/crypto/sha256.c
index d43bc39ab05e..099cd11f83c1 100644
--- a/lib/crypto/sha256.c
+++ b/lib/crypto/sha256.c
@@ -202,7 +202,6 @@ static void sha256_transform(u32 *state, const u8 *input)
 	state[4] += e; state[5] += f; state[6] += g; state[7] += h;
 
 	/* clear any sensitive info... */
-	a = b = c = d = e = f = g = h = t1 = t2 = 0;
 	memzero_explicit(W, 64 * sizeof(u32));
 }
 
-- 
2.26.2

