@@ -146,6 +146,17 @@ static void gf128mul_x8_lle(be128 *x)
x->a = cpu_to_be64((a >> 8) ^ (_tt << 48));
}
+/* time invariant version of gf128mul_x8_lle */
+static void gf128mul_x8_lle_ti(be128 *x)
+{
+ u64 a = be64_to_cpu(x->a);
+ u64 b = be64_to_cpu(x->b);
+ u64 _tt = xda_le(b & 0xff); /* avoid table lookup */
+
+ x->b = cpu_to_be64((b >> 8) | (a << 56));
+ x->a = cpu_to_be64((a >> 8) ^ (_tt << 48));
+}
+
static void gf128mul_x8_bbe(be128 *x)
{
u64 a = be64_to_cpu(x->a);
@@ -169,38 +180,47 @@ EXPORT_SYMBOL(gf128mul_x8_ble);
void gf128mul_lle(be128 *r, const be128 *b)
{
- be128 p[8];
+ /*
+ * The p array should be aligned to twice the size of its element type,
+ * so that every even/odd pair is guaranteed to share a cacheline
+ * (assuming a cacheline size of 32 bytes or more, which is by far the
+ * most common). This ensures that each be128_xor() call in the loop
+ * takes the same amount of time regardless of the value of 'ch', which
+ * is derived from function parameter 'b', which is commonly used as a
+ * key, e.g., for GHASH. The odd array elements are all set to zero,
+ * making each be128_xor() a NOP if its associated bit in 'ch' is not
+ * set, and this is equivalent to calling be128_xor() conditionally.
+ * This approach aims to avoid leaking information about such keys
+ * through execution time variances.
+ *
+ * Unfortunately, __aligned(16) or higher does not work on x86 for
+ * variables on the stack so we need to perform the alignment by hand.
+ */
+ be128 array[16 + 3] = {};
+ be128 *p = PTR_ALIGN(&array[0], 2 * sizeof(be128));
int i;
p[0] = *r;
for (i = 0; i < 7; ++i)
- gf128mul_x_lle(&p[i + 1], &p[i]);
+ gf128mul_x_lle(&p[2 * i + 2], &p[2 * i]);
memset(r, 0, sizeof(*r));
for (i = 0;;) {
u8 ch = ((u8 *)b)[15 - i];
- if (ch & 0x80)
- be128_xor(r, r, &p[0]);
- if (ch & 0x40)
- be128_xor(r, r, &p[1]);
- if (ch & 0x20)
- be128_xor(r, r, &p[2]);
- if (ch & 0x10)
- be128_xor(r, r, &p[3]);
- if (ch & 0x08)
- be128_xor(r, r, &p[4]);
- if (ch & 0x04)
- be128_xor(r, r, &p[5]);
- if (ch & 0x02)
- be128_xor(r, r, &p[6]);
- if (ch & 0x01)
- be128_xor(r, r, &p[7]);
+ be128_xor(r, r, &p[ 0 + !(ch & 0x80)]);
+ be128_xor(r, r, &p[ 2 + !(ch & 0x40)]);
+ be128_xor(r, r, &p[ 4 + !(ch & 0x20)]);
+ be128_xor(r, r, &p[ 6 + !(ch & 0x10)]);
+ be128_xor(r, r, &p[ 8 + !(ch & 0x08)]);
+ be128_xor(r, r, &p[10 + !(ch & 0x04)]);
+ be128_xor(r, r, &p[12 + !(ch & 0x02)]);
+ be128_xor(r, r, &p[14 + !(ch & 0x01)]);
if (++i >= 16)
break;
- gf128mul_x8_lle(r);
+ gf128mul_x8_lle_ti(r); /* use the time invariant version */
}
}
EXPORT_SYMBOL(gf128mul_lle);
The gf128mul library has different variants with different memory/performance tradeoffs, where the faster ones use 4k or 64k lookup tables precomputed at runtime, which are based on one of the multiplication factors, which is commonly the key for keyed hash algorithms such as GHASH. The slowest variant is gf128_mul_lle() [and its bbe/ble counterparts], which does not use precomputed lookup tables, but it still relies on a single u16[256] lookup table which is input independent. The use of such a table may cause the execution time of gf128_mul_lle() to correlate with the value of the inputs, which is generally something that must be avoided for cryptographic algorithms. On top of that, the function uses a sequence of if () statements that conditionally invoke be128_xor() based on which bits are set in the second argument of the function, which is usually a pointer to the multiplication factor that represents the key. In order to remove the correlation between the execution time of gf128_mul_lle() and the value of its inputs, let's address the identified shortcomings: - add a time invariant version of gf128mul_x8_lle() that replaces the table lookup with the expression that is used at compile time to populate the lookup table; - make the invocations of be128_xor() unconditional, but pass a zero vector as the third argument if the associated bit in the key is cleared. The resulting code is likely to be significantly slower. However, given that this is the slowest version already, making it even slower in order to make it more secure is assumed to be justified. The bbe and ble counterparts could receive the same treatment, but the former is never used anywhere in the kernel, and the latter is only used in the driver for a asynchronous crypto h/w accelerator (Chelsio), where timing variances are unlikely to matter. Signed-off-by: Ard Biesheuvel <ardb@kernel.org> --- lib/crypto/gf128mul.c | 58 +++++++++++++------- 1 file changed, 39 insertions(+), 19 deletions(-)