[PATCH 2/4] selftests/sgx: Fix Q1 and Q2 calculation in sigstruct.c

[Jarkko Sakkinen <jarkko@kernel.org>]

From: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>

Q1 and Q2 are numbers with *maximum* length of 384 bytes. If the calculated
length of Q1 and Q2 is less than 384 bytes, things will go wrong.

E.g. if Q2 is 383 bytes, then

1. The bytes of q2 are copied to sigstruct->q2 in calc_q1q2().
2. The entire sigstruct->q2 is reversed, which results it being
   256 * Q2, given that the last byte of sigstruct->q2 is added
   to before the bytes given by calc_q1q2().

Either change in key or measurement can trigger the bug. E.g. an unmeasured
heap could cause a devastating change in Q1 or Q2.

Reverse exactly the bytes of Q1 and Q2 in calc_q1q2() before returning to
the caller.

Fixes: 2adcba79e69d ("selftests/x86: Add a selftest for SGX")
Link: https://lore.kernel.org/linux-sgx/20210301051836.30738-1-tianjia.zhang@linux.alibaba.com/
Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
---
The original patch did a bad job explaining the code change but it
turned out making sense. I wrote a new description.

v3:
- Adjusted the fixes tag to refer to the correct commit

v2:
- Added a fixes tag.

 tools/testing/selftests/sgx/sigstruct.c | 41 +++++++++++++------------
 1 file changed, 21 insertions(+), 20 deletions(-)

diff --git a/tools/testing/selftests/sgx/sigstruct.c b/tools/testing/selftests/sgx/sigstruct.c
index dee7a3d6c5a5..92bbc5a15c39 100644
--- a/tools/testing/selftests/sgx/sigstruct.c
+++ b/tools/testing/selftests/sgx/sigstruct.c
@@ -55,10 +55,27 @@ static bool alloc_q1q2_ctx(const uint8_t *s, const uint8_t *m,
 	return true;
 }
 
+static void reverse_bytes(void *data, int length)
+{
+	int i = 0;
+	int j = length - 1;
+	uint8_t temp;
+	uint8_t *ptr = data;
+
+	while (i < j) {
+		temp = ptr[i];
+		ptr[i] = ptr[j];
+		ptr[j] = temp;
+		i++;
+		j--;
+	}
+}
+
 static bool calc_q1q2(const uint8_t *s, const uint8_t *m, uint8_t *q1,
 		      uint8_t *q2)
 {
 	struct q1q2_ctx ctx;
+	int len;
 
 	if (!alloc_q1q2_ctx(s, m, &ctx)) {
 		fprintf(stderr, "Not enough memory for Q1Q2 calculation\n");
@@ -89,8 +106,10 @@ static bool calc_q1q2(const uint8_t *s, const uint8_t *m, uint8_t *q1,
 		goto out;
 	}
 
-	BN_bn2bin(ctx.q1, q1);
-	BN_bn2bin(ctx.q2, q2);
+	len = BN_bn2bin(ctx.q1, q1);
+	reverse_bytes(q1, len);
+	len = BN_bn2bin(ctx.q2, q2);
+	reverse_bytes(q2, len);
 
 	free_q1q2_ctx(&ctx);
 	return true;
@@ -152,22 +171,6 @@ static RSA *gen_sign_key(void)
 	return key;
 }
 
-static void reverse_bytes(void *data, int length)
-{
-	int i = 0;
-	int j = length - 1;
-	uint8_t temp;
-	uint8_t *ptr = data;
-
-	while (i < j) {
-		temp = ptr[i];
-		ptr[i] = ptr[j];
-		ptr[j] = temp;
-		i++;
-		j--;
-	}
-}
-
 enum mrtags {
 	MRECREATE = 0x0045544145524345,
 	MREADD = 0x0000000044444145,
@@ -367,8 +370,6 @@ bool encl_measure(struct encl *encl)
 	/* BE -> LE */
 	reverse_bytes(sigstruct->signature, SGX_MODULUS_SIZE);
 	reverse_bytes(sigstruct->modulus, SGX_MODULUS_SIZE);
-	reverse_bytes(sigstruct->q1, SGX_MODULUS_SIZE);
-	reverse_bytes(sigstruct->q2, SGX_MODULUS_SIZE);
 
 	EVP_MD_CTX_destroy(ctx);
 	RSA_free(key);
-- 
2.32.0