linux-kernel - [PATCH v2 2/6] crypto: lib/sha256 - Don't clear temporary variables

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Message-Id: <20201020203957.3512851-3-nivedita@alum.mit.edu>
Date:   Tue, 20 Oct 2020 16:39:53 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "linux-crypto@...r.kernel.org" <linux-crypto@...r.kernel.org>,
        David Laight <David.Laight@...LAB.COM>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH v2 2/6] crypto: lib/sha256 - Don't clear temporary variables

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

These variables shouldn't be very sensitive: t1/t2 can be calculated
from a through h, so they don't reveal any additional information.
Knowing a through h is equivalent to knowing one 64-byte block's SHA256
hash (with non-standard initial value) which, assuming SHA256 is secure,
doesn't reveal any information about the input.

Signed-off-by: Arvind Sankar <nivedita@...m.mit.edu>
---
 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