diff --git a/library/bignum_core.c b/library/bignum_core.c
index 1e80a5bf5d..e020da100e 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -1094,11 +1094,19 @@ void mbedtls_mpi_core_gcd_modinv_odd(mbedtls_mpi_uint *G,
      * Initial values:
      * u, v = A, N
      * q, r = 0, 1
+     *
+     * We only write to G (aka v) after reading from inputs (A and N), which
+     * allows aliasing, except with N when I != NULL, as then we'll be operating
+     * mod N on q and r later - see the public documentation.
+     *
+     * Also avoid possible UB with memcpy when src == dst.
      */
     memcpy(u, A, A_limbs * ciL);
     memset((char *) u + A_limbs * ciL, 0, (N_limbs - A_limbs) * ciL);
 
-    memcpy(v, N, N_limbs * ciL);
+    if (v != N) {
+        memcpy(v, N, N_limbs * ciL);
+    }
 
     if (I != NULL) {
         memset(q, 0, N_limbs * ciL);
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 29e05cd3c9..1589c34640 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -826,7 +826,9 @@ void mbedtls_mpi_core_from_mont_rep(mbedtls_mpi_uint *X,
  *
  * Requires N to be odd, and 0 <= A <= N.
  *
- * None of the parameters may alias or overlap another.
+ * When I == NULL (computing only the GCD), G may alias A or N.
+ * When I != NULL (computing the modular inverse), G or I may alias A
+ * but none of them may alias N (the modulus).
  *
  * \param[out]    G       The GCD of \p A and \p N.
  *                        Must have the same number of limbs as \p N.
diff --git a/tests/suites/test_suite_bignum_core.function b/tests/suites/test_suite_bignum_core.function
index 81c2242997..93782c1331 100644
--- a/tests/suites/test_suite_bignum_core.function
+++ b/tests/suites/test_suite_bignum_core.function
@@ -1416,6 +1416,38 @@ void mpi_core_gcd_modinv_odd(char *input_A, char *input_N,
     mbedtls_free(T);
     T = NULL;
 
+    /* GCD only, G aliased to N */
+    TEST_CALLOC(G, N_limbs);
+    memcpy(G, N, bytes);
+
+    TEST_CALLOC(T, 4 * N_limbs);
+    memset(T, 'T', bytes);
+
+    mbedtls_mpi_core_gcd_modinv_odd(G, NULL, A, A_limbs, G, N_limbs, T);
+    TEST_CF_PUBLIC(G, bytes);
+    TEST_MEMORY_COMPARE(G, bytes, exp_G, bytes);
+
+    mbedtls_free(G);
+    G = NULL;
+    mbedtls_free(T);
+    T = NULL;
+
+    /* GCD only, G aliased to A (hence A_limbs = N_limbs) */
+    TEST_CALLOC(G, N_limbs);
+    memcpy(G, A, A_limbs * sizeof(mbedtls_mpi_uint));
+
+    TEST_CALLOC(T, 4 * N_limbs);
+    memset(T, 'T', bytes);
+
+    mbedtls_mpi_core_gcd_modinv_odd(G, NULL, G, N_limbs, N, N_limbs, T);
+    TEST_CF_PUBLIC(G, bytes);
+    TEST_MEMORY_COMPARE(G, bytes, exp_G, bytes);
+
+    mbedtls_free(G);
+    G = NULL;
+    mbedtls_free(T);
+    T = NULL;
+
     /*
      * Test GCD + modinv
      */
@@ -1437,6 +1469,65 @@ void mpi_core_gcd_modinv_odd(char *input_A, char *input_N,
         TEST_MEMORY_COMPARE(I, bytes, exp_I, bytes);
     }
 
+    mbedtls_free(G);
+    G = NULL;
+    mbedtls_free(I);
+    I = NULL;
+    mbedtls_free(T);
+    T = NULL;
+
+    /* GCD + modinv, G aliased to A */
+    TEST_CALLOC(G, N_limbs);
+    memcpy(G, A, A_limbs * sizeof(mbedtls_mpi_uint));
+
+    TEST_CALLOC(I, N_limbs);
+    memset(I, 'I', bytes);
+
+    TEST_CALLOC(T, 5 * N_limbs);
+    memset(T, 'T', bytes);
+
+    mbedtls_mpi_core_gcd_modinv_odd(G, I, G, N_limbs, N, N_limbs, T);
+
+    TEST_CF_PUBLIC(G, bytes);
+    TEST_MEMORY_COMPARE(G, bytes, exp_G, bytes);
+    if (got_I) {
+        TEST_CF_PUBLIC(I, bytes);
+        TEST_MEMORY_COMPARE(I, bytes, exp_I, bytes);
+    }
+
+    mbedtls_free(G);
+    G = NULL;
+    mbedtls_free(I);
+    I = NULL;
+    mbedtls_free(T);
+    T = NULL;
+
+    /* GCD + modinv, I aliased to A */
+    TEST_CALLOC(G, N_limbs);
+    memset(G, 'G', bytes);
+
+    TEST_CALLOC(I, N_limbs);
+    memcpy(I, A, A_limbs * sizeof(mbedtls_mpi_uint));
+
+    TEST_CALLOC(T, 5 * N_limbs);
+    memset(T, 'T', bytes);
+
+    mbedtls_mpi_core_gcd_modinv_odd(G, I, I, N_limbs, N, N_limbs, T);
+
+    TEST_CF_PUBLIC(G, bytes);
+    TEST_MEMORY_COMPARE(G, bytes, exp_G, bytes);
+    if (got_I) {
+        TEST_CF_PUBLIC(I, bytes);
+        TEST_MEMORY_COMPARE(I, bytes, exp_I, bytes);
+    }
+
+    mbedtls_free(G);
+    G = NULL;
+    mbedtls_free(I);
+    I = NULL;
+    mbedtls_free(T);
+    T = NULL;
+
 exit:
     mbedtls_free(A);
     mbedtls_free(N);