lib/crypto: gf128hash: Hide selection mask values from compilers

Hide selection mask values from compilers, so that they cannot see that they are used to select between two values and potentially generate a secret-dependent branch. This is not known to be happening here, but we might as well be extra careful.

Signed-off-by Eric Biggers <[email protected]>

---
 lib/crypto/gf128hash.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

lib/crypto/gf128hash.c +13 -4

index 8dcdf5ec98be..5b8ae98cafed 100644
@@ -88,10 +88,18 @@ static void clmul64(u64 a, u64 b, u64 *out_lo, u64 *out_hi)
 		  (a2 * (u128)b1) ^ (a3 * (u128)b0);
 
 	/* Multiply the low 4 bits of @a by @b. */
-	u64 e0 = -(a & 1) & b;
-	u64 e1 = -((a >> 1) & 1) & b;
-	u64 e2 = -((a >> 2) & 1) & b;
-	u64 e3 = -((a >> 3) & 1) & b;
+	u64 e0 = -(a & 1);
+	u64 e1 = -((a >> 1) & 1);
+	u64 e2 = -((a >> 2) & 1);
+	u64 e3 = -((a >> 3) & 1);
+	OPTIMIZER_HIDE_VAR(e0);
+	OPTIMIZER_HIDE_VAR(e1);
+	OPTIMIZER_HIDE_VAR(e2);
+	OPTIMIZER_HIDE_VAR(e3);
+	e0 &= b;
+	e1 &= b;
+	e2 &= b;
+	e3 &= b;
 	u64 extra_lo = e0 ^ (e1 << 1) ^ (e2 << 2) ^ (e3 << 3);
 	u64 extra_hi = (e1 >> 63) ^ (e2 >> 62) ^ (e3 >> 61);
 

@@ -245,6 +253,7 @@ static void __maybe_unused 
 ghash_key_to_polyval(const u8 in[GHASH_BLOCK_SIZE],
 	u64 lo = get_unaligned_be64(&in[8]);
 	u64 mask = (s64)hi >> 63;
 
+	OPTIMIZER_HIDE_VAR(mask);
 	hi = (hi << 1) ^ (lo >> 63) ^ (mask & ((u64)0xc2 << 56));
 	lo = (lo << 1) ^ (mask & 1);

out->lo = cpu_to_le64(lo);

base-commit: 2a500a04d6f969318c05f46aedb6e73fc1963a00

Signature · 1 line

2.56.0

Hide selection mask values from compilers, so that they cannot see that they are used to select between two values and potentially generate a secret-dependent branch. This is not known to be happening here, but we might as well be extra careful.

Signed-off-by Eric Biggers <[email protected]>

---
 lib/crypto/gf128hash.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)