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From:   Mahmoud Adam <mngyadam@amazon.com>
To:     <herbert@gondor.apana.org.au>
CC:     <davem@davemloft.net>, <linux-crypto@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, Mahmoud Adam <mngyadam@amazon.com>
Subject: [PATCH] crypto: rsa - allow only odd e and restrict value in FIPS mode
Date:   Tue, 13 Jun 2023 16:17:31 +0000
Message-ID: <20230613161731.74081-1-mngyadam@amazon.com>
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check if rsa public exponent is odd and check its value is between
2^16 < e < 2^256.

FIPS 186-5 DSS (page 35)[1] specify that:
1. The public exponent e shall be selected with the following constraints:
  (a) The public verification exponent e shall be selected prior to
  generating the primes, p and q, and the private signature exponent
  d.
  (b) The exponent e shall be an odd positive integer such that:
   2^16 < e < 2^256.

[1] https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.186-5.pdf

Signed-off-by: Mahmoud Adam <mngyadam@amazon.com>
---
 crypto/rsa.c | 36 ++++++++++++++++++++++++++++++++++++
 1 file changed, 36 insertions(+)

diff --git a/crypto/rsa.c b/crypto/rsa.c
index c50f2d2a4d06..c79613cdce6e 100644
--- a/crypto/rsa.c
+++ b/crypto/rsa.c
@@ -205,6 +205,32 @@ static int rsa_check_key_length(unsigned int len)
 	return -EINVAL;
 }
 
+static int rsa_check_exponent_fips(MPI e)
+{
+	MPI e_max = NULL;
+
+	/* check if odd */
+	if (!mpi_test_bit(e, 0)) {
+		return -EINVAL;
+	}
+
+	/* check if 2^16 < e < 2^256. */
+	if (mpi_cmp_ui(e, 65536) <= 0) {
+		return -EINVAL;
+	}
+
+	e_max = mpi_alloc(0);
+	mpi_set_bit(e_max, 256);
+
+	if (mpi_cmp(e, e_max) >= 0) {
+		mpi_free(e_max);
+		return -EINVAL;
+	}
+
+	mpi_free(e_max);
+	return 0;
+}
+
 static int rsa_set_pub_key(struct crypto_akcipher *tfm, const void *key,
 			   unsigned int keylen)
 {
@@ -232,6 +258,11 @@ static int rsa_set_pub_key(struct crypto_akcipher *tfm, const void *key,
 		return -EINVAL;
 	}
 
+	if (fips_enabled && rsa_check_exponent_fips(mpi_key->e)) {
+		rsa_free_mpi_key(mpi_key);
+		return -EINVAL;
+	}
+
 	return 0;
 
 err:
@@ -290,6 +321,11 @@ static int rsa_set_priv_key(struct crypto_akcipher *tfm, const void *key,
 		return -EINVAL;
 	}
 
+	if (fips_enabled && rsa_check_exponent_fips(mpi_key->e)) {
+		rsa_free_mpi_key(mpi_key);
+		return -EINVAL;
+	}
+
 	return 0;
 
 err:
-- 
2.40.1