Description: Fix for CVE-2021-3580 - potential crash on invalid input to the RSA decryption functions.
Origin: upstream, https://git.lysator.liu.se/nettle/nettle/-/compare/7a5f86321f4c67d7219aa87ea4e2ddca677d7378...0ad0b5df315665250dfdaa4a1e087f4799edaefe
Bug-RedHat: https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2021-3580

--- a/pkcs1-sec-decrypt.c
+++ b/pkcs1-sec-decrypt.c
@@ -64,7 +64,9 @@ _pkcs1_sec_decrypt (size_t length, uint8
   volatile int ok;
   size_t i, t;
 
-  assert (padded_message_length >= length);
+  /* Message independent branch */
+  if (length + 11 > padded_message_length)
+    return 0;
 
   t = padded_message_length - length - 1;
 
--- a/rsa-decrypt-tr.c
+++ b/rsa-decrypt-tr.c
@@ -52,14 +52,17 @@ rsa_decrypt_tr(const struct rsa_public_k
   mp_size_t key_limb_size;
   int res;
 
-  key_limb_size = NETTLE_OCTET_SIZE_TO_LIMB_SIZE(key->size);
+  /* First check that input is in range. */
+  if (mpz_sgn (gibberish) < 0 || mpz_cmp (gibberish, pub->n) >= 0)
+    return 0;
+
+  key_limb_size = mpz_size(pub->n);
 
   TMP_GMP_ALLOC (m, key_limb_size);
   TMP_GMP_ALLOC (em, key->size);
+  mpz_limbs_copy(m, gibberish, key_limb_size);
 
-  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m,
-				  mpz_limbs_read(gibberish),
-				  mpz_size(gibberish));
+  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m, m);
 
   mpn_get_base256 (em, key->size, m, key_limb_size);
 
--- a/rsa-decrypt.c
+++ b/rsa-decrypt.c
@@ -48,6 +48,16 @@ rsa_decrypt(const struct rsa_private_key
   int res;
 
   mpz_init(m);
+
+  /* First check that input is in range. Since we don't have the
+     public key available here, we need to reconstruct n. */
+  mpz_mul (m, key->p, key->q);
+  if (mpz_sgn (gibberish) < 0 || mpz_cmp (gibberish, m) >= 0)
+    {
+      mpz_clear (m);
+      return 0;
+    }
+
   rsa_compute_root(key, m, gibberish);
 
   res = pkcs1_decrypt (key->size, m, length, message);
--- a/rsa-internal.h
+++ b/rsa-internal.h
@@ -53,11 +53,11 @@ _rsa_sec_compute_root(const struct rsa_p
                       mp_limb_t *scratch);
 
 /* Safe side-channel silent variant, using RSA blinding, and checking the
- * result after CRT. */
+ * result after CRT. In-place calls, with x == m, is allowed. */
 int
 _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
 			 const struct rsa_private_key *key,
 			 void *random_ctx, nettle_random_func *random,
-			 mp_limb_t *x, const mp_limb_t *m, size_t mn);
+			 mp_limb_t *x, const mp_limb_t *m);
 
 #endif /* NETTLE_RSA_INTERNAL_H_INCLUDED */
--- a/rsa-sec-decrypt.c
+++ b/rsa-sec-decrypt.c
@@ -54,12 +54,19 @@ rsa_sec_decrypt(const struct rsa_public_
   TMP_GMP_DECL (em, uint8_t);
   int res;
 
+  /* First check that input is in range. */
+  if (mpz_sgn (gibberish) < 0 || mpz_cmp (gibberish, pub->n) >= 0)
+    return 0;
+
   TMP_GMP_ALLOC (m, mpz_size(pub->n));
   TMP_GMP_ALLOC (em, key->size);
 
-  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m,
-				  mpz_limbs_read(gibberish),
-				  mpz_size(gibberish));
+  /* We need a copy because m can be shorter than key_size,
+   * but _rsa_sec_compute_root_tr expect all inputs to be
+   * normalized to a key_size long buffer length */
+  mpz_limbs_copy(m, gibberish, mpz_size(pub->n));
+
+  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, m, m);
 
   mpn_get_base256 (em, key->size, m, mpz_size(pub->n));
 
--- a/rsa-sign-tr.c
+++ b/rsa-sign-tr.c
@@ -131,35 +131,34 @@ int
 _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
 			 const struct rsa_private_key *key,
 			 void *random_ctx, nettle_random_func *random,
-			 mp_limb_t *x, const mp_limb_t *m, size_t mn)
+			 mp_limb_t *x, const mp_limb_t *m)
 {
+  mp_size_t nn;
   mpz_t mz;
   mpz_t xz;
   int res;
 
-  mpz_init(mz);
   mpz_init(xz);
 
-  mpn_copyi(mpz_limbs_write(mz, mn), m, mn);
-  mpz_limbs_finish(mz, mn);
+  nn = mpz_size (pub->n);
 
-  res = rsa_compute_root_tr(pub, key, random_ctx, random, xz, mz);
+  res = rsa_compute_root_tr(pub, key, random_ctx, random, xz,
+			    mpz_roinit_n(mz, m, nn));
 
   if (res)
-    mpz_limbs_copy(x, xz, mpz_size(pub->n));
+    mpz_limbs_copy(x, xz, nn);
 
-  mpz_clear(mz);
   mpz_clear(xz);
   return res;
 }
 #else
 /* Blinds m, by computing c = m r^e (mod n), for a random r. Also
-   returns the inverse (ri), for use by rsa_unblind. */
+   returns the inverse (ri), for use by rsa_unblind. Must have c != m,
+   no in-place operation.*/
 static void
 rsa_sec_blind (const struct rsa_public_key *pub,
                void *random_ctx, nettle_random_func *random,
-               mp_limb_t *c, mp_limb_t *ri, const mp_limb_t *m,
-               mp_size_t mn)
+               mp_limb_t *c, mp_limb_t *ri, const mp_limb_t *m)
 {
   const mp_limb_t *ep = mpz_limbs_read (pub->e);
   const mp_limb_t *np = mpz_limbs_read (pub->n);
@@ -177,15 +176,15 @@ rsa_sec_blind (const struct rsa_public_k
 
   /* c = m*(r^e) mod n */
   itch = mpn_sec_powm_itch(nn, ebn, nn);
-  i2 = mpn_sec_mul_itch(nn, mn);
+  i2 = mpn_sec_mul_itch(nn, nn);
   itch = MAX(itch, i2);
-  i2 = mpn_sec_div_r_itch(nn + mn, nn);
+  i2 = mpn_sec_div_r_itch(2*nn, nn);
   itch = MAX(itch, i2);
   i2 = mpn_sec_invert_itch(nn);
   itch = MAX(itch, i2);
 
-  TMP_GMP_ALLOC (tp, nn + mn + itch);
-  scratch = tp + nn + mn;
+  TMP_GMP_ALLOC (tp, 2*nn  + itch);
+  scratch = tp + 2*nn;
 
   /* ri = r^(-1) */
   do
@@ -198,9 +197,8 @@ rsa_sec_blind (const struct rsa_public_k
   while (!mpn_sec_invert (ri, tp, np, nn, 2 * nn * GMP_NUMB_BITS, scratch));
 
   mpn_sec_powm (c, rp, nn, ep, ebn, np, nn, scratch);
-  /* normally mn == nn, but m can be smaller in some cases */
-  mpn_sec_mul (tp, c, nn, m, mn, scratch);
-  mpn_sec_div_r (tp, nn + mn, np, nn, scratch);
+  mpn_sec_mul (tp, c, nn, m, nn, scratch);
+  mpn_sec_div_r (tp, 2*nn, np, nn, scratch);
   mpn_copyi(c, tp, nn);
 
   TMP_GMP_FREE (r);
@@ -208,7 +206,7 @@ rsa_sec_blind (const struct rsa_public_k
   TMP_GMP_FREE (tp);
 }
 
-/* m = c ri mod n */
+/* m = c ri mod n. Allows x == c. */
 static void
 rsa_sec_unblind (const struct rsa_public_key *pub,
                  mp_limb_t *x, mp_limb_t *ri, const mp_limb_t *c)
@@ -298,7 +296,7 @@ int
 _rsa_sec_compute_root_tr(const struct rsa_public_key *pub,
 			 const struct rsa_private_key *key,
 			 void *random_ctx, nettle_random_func *random,
-			 mp_limb_t *x, const mp_limb_t *m, size_t mn)
+			 mp_limb_t *x, const mp_limb_t *m)
 {
   TMP_GMP_DECL (c, mp_limb_t);
   TMP_GMP_DECL (ri, mp_limb_t);
@@ -306,7 +304,7 @@ _rsa_sec_compute_root_tr(const struct rs
   size_t key_limb_size;
   int ret;
 
-  key_limb_size = NETTLE_OCTET_SIZE_TO_LIMB_SIZE(key->size);
+  key_limb_size = mpz_size(pub->n);
 
   /* mpz_powm_sec handles only odd moduli. If p, q or n is even, the
      key is invalid and rejected by rsa_private_key_prepare. However,
@@ -320,19 +318,18 @@ _rsa_sec_compute_root_tr(const struct rs
     }
 
   assert(mpz_size(pub->n) == key_limb_size);
-  assert(mn <= key_limb_size);
 
   TMP_GMP_ALLOC (c, key_limb_size);
   TMP_GMP_ALLOC (ri, key_limb_size);
   TMP_GMP_ALLOC (scratch, _rsa_sec_compute_root_itch(key));
 
-  rsa_sec_blind (pub, random_ctx, random, x, ri, m, mn);
+  rsa_sec_blind (pub, random_ctx, random, c, ri, m);
 
-  _rsa_sec_compute_root(key, c, x, scratch);
+  _rsa_sec_compute_root(key, x, c, scratch);
 
-  ret = rsa_sec_check_root(pub, c, x);
+  ret = rsa_sec_check_root(pub, x, c);
 
-  rsa_sec_unblind(pub, x, ri, c);
+  rsa_sec_unblind(pub, x, ri, x);
 
   cnd_mpn_zero(1 - ret, x, key_limb_size);
 
@@ -356,17 +353,17 @@ rsa_compute_root_tr(const struct rsa_pub
 		    mpz_t x, const mpz_t m)
 {
   TMP_GMP_DECL (l, mp_limb_t);
+  mp_size_t nn = mpz_size(pub->n);
   int res;
 
-  mp_size_t l_size = NETTLE_OCTET_SIZE_TO_LIMB_SIZE(key->size);
-  TMP_GMP_ALLOC (l, l_size);
+  TMP_GMP_ALLOC (l, nn);
+  mpz_limbs_copy(l, m, nn);
 
-  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, l,
-				  mpz_limbs_read(m), mpz_size(m));
+  res = _rsa_sec_compute_root_tr (pub, key, random_ctx, random, l, l);
   if (res) {
-    mp_limb_t *xp = mpz_limbs_write (x, l_size);
-    mpn_copyi (xp, l, l_size);
-    mpz_limbs_finish (x, l_size);
+    mp_limb_t *xp = mpz_limbs_write (x, nn);
+    mpn_copyi (xp, l, nn);
+    mpz_limbs_finish (x, nn);
   }
 
   TMP_GMP_FREE (l);
--- a/rsa.h
+++ b/rsa.h
@@ -433,13 +433,14 @@ rsa_sec_decrypt(const struct rsa_public_
 	        size_t length, uint8_t *message,
 	        const mpz_t gibberish);
 
-/* Compute x, the e:th root of m. Calling it with x == m is allowed. */
+/* Compute x, the e:th root of m. Calling it with x == m is allowed.
+   It is required that 0 <= m < n. */
 void
 rsa_compute_root(const struct rsa_private_key *key,
 		 mpz_t x, const mpz_t m);
 
 /* Safer variant, using RSA blinding, and checking the result after
-   CRT. */
+   CRT. It is required that 0 <= m < n. */
 int
 rsa_compute_root_tr(const struct rsa_public_key *pub,
 		    const struct rsa_private_key *key,
--- a/testsuite/rsa-encrypt-test.c
+++ b/testsuite/rsa-encrypt-test.c
@@ -19,10 +19,12 @@ test_main(void)
   uint8_t after;
 
   mpz_t gibberish;
+  mpz_t bad_input;
 
   rsa_private_key_init(&key);
   rsa_public_key_init(&pub);
   mpz_init(gibberish);
+  mpz_init(bad_input);
 
   knuth_lfib_init(&lfib, 17);
   
@@ -101,6 +103,42 @@ test_main(void)
   ASSERT(decrypted[decrypted_length] == after);
   ASSERT(decrypted[0] == 'A');
 
+  /* Test zero input. */
+  mpz_set_ui (bad_input, 0);
+  decrypted_length = msg_length;
+  ASSERT(!rsa_decrypt(&key, &decrypted_length, decrypted, bad_input));
+  ASSERT(!rsa_decrypt_tr(&pub, &key,
+			 &lfib, (nettle_random_func *) knuth_lfib_random,
+			 &decrypted_length, decrypted, bad_input));
+  ASSERT(!rsa_sec_decrypt(&pub, &key,
+			  &lfib, (nettle_random_func *) knuth_lfib_random,
+			  decrypted_length, decrypted, bad_input));
+  ASSERT(decrypted_length == msg_length);
+
+  /* Test input that is slightly larger than n */
+  mpz_add(bad_input, gibberish, pub.n);
+  decrypted_length = msg_length;
+  ASSERT(!rsa_decrypt(&key, &decrypted_length, decrypted, bad_input));
+  ASSERT(!rsa_decrypt_tr(&pub, &key,
+			 &lfib, (nettle_random_func *) knuth_lfib_random,
+			 &decrypted_length, decrypted, bad_input));
+  ASSERT(!rsa_sec_decrypt(&pub, &key,
+			  &lfib, (nettle_random_func *) knuth_lfib_random,
+			  decrypted_length, decrypted, bad_input));
+  ASSERT(decrypted_length == msg_length);
+
+  /* Test input that is considerably larger than n */
+  mpz_mul_2exp (bad_input, pub.n, 100);
+  mpz_add (bad_input, bad_input, gibberish);
+  decrypted_length = msg_length;
+  ASSERT(!rsa_decrypt(&key, &decrypted_length, decrypted, bad_input));
+  ASSERT(!rsa_decrypt_tr(&pub, &key,
+			 &lfib, (nettle_random_func *) knuth_lfib_random,
+			 &decrypted_length, decrypted, bad_input));
+  ASSERT(!rsa_sec_decrypt(&pub, &key,
+			  &lfib, (nettle_random_func *) knuth_lfib_random,
+			  decrypted_length, decrypted, bad_input));
+  ASSERT(decrypted_length == msg_length);
 
   /* Test invalid key. */
   mpz_add_ui (key.q, key.q, 2);
@@ -112,6 +150,6 @@ test_main(void)
   rsa_private_key_clear(&key);
   rsa_public_key_clear(&pub);
   mpz_clear(gibberish);
+  mpz_clear(bad_input);
   free(decrypted);
 }
-  
--- a/testsuite/rsa-sec-decrypt-test.c
+++ b/testsuite/rsa-sec-decrypt-test.c
@@ -55,6 +55,7 @@ rsa_decrypt_for_test(const struct rsa_pu
 #endif
 
 #define PAYLOAD_SIZE 50
+#define DECRYPTED_SIZE 256
 void
 test_main(void)
 {
@@ -63,7 +64,7 @@ test_main(void)
   struct knuth_lfib_ctx random_ctx;
 
   uint8_t plaintext[PAYLOAD_SIZE];
-  uint8_t decrypted[PAYLOAD_SIZE];
+  uint8_t decrypted[DECRYPTED_SIZE];
   uint8_t verifybad[PAYLOAD_SIZE];
   unsigned n_size = 1024;
   mpz_t gibberish;
@@ -98,6 +99,20 @@ test_main(void)
                                     PAYLOAD_SIZE, decrypted, gibberish) == 1);
       ASSERT (MEMEQ (PAYLOAD_SIZE, plaintext, decrypted));
 
+      ASSERT (pub.size > 10);
+      ASSERT (pub.size <= DECRYPTED_SIZE);
+
+      /* Check that too large message length is rejected, largest
+	 valid size is pub.size - 11. */
+      ASSERT (!rsa_decrypt_for_test (&pub, &key, &random_ctx,
+				     (nettle_random_func *) knuth_lfib_random,
+				     pub.size - 10, decrypted, gibberish));
+
+      /* This case used to result in arithmetic underflow and a crash. */
+      ASSERT (!rsa_decrypt_for_test (&pub, &key, &random_ctx,
+				     (nettle_random_func *) knuth_lfib_random,
+				     pub.size, decrypted, gibberish));
+
       /* bad one */
       memcpy(decrypted, verifybad, PAYLOAD_SIZE);
       nettle_mpz_random_size(garbage, &random_ctx,

