diff --git a/ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt b/ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt
new file mode 100644
index 0000000000..564f9ac1b6
--- /dev/null
+++ b/ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt
@@ -0,0 +1,5 @@
+Bugfix
+   * Fix mbedtls_mpi_sub_abs() to account for the possibility that the output
+     pointer could equal the first input pointer and if so to skip a memcpy()
+     call that would be redundant.  Reported by Pascal Cuoq using TrustInSoft
+     Analyzer in #6701; observed independently by Aaron Ucko under Valgrind.
diff --git a/library/bignum.c b/library/bignum.c
index 9bc1c2d43f..41b3a26914 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1009,7 +1009,7 @@ int mbedtls_mpi_sub_abs(mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
     /* Set the high limbs of X to match A. Don't touch the lower limbs
      * because X might be aliased to B, and we must not overwrite the
      * significant digits of B. */
-    if (A->n > n) {
+    if (A->n > n && A != X) {
         memcpy(X->p + n, A->p + n, (A->n - n) * ciL);
     }
     if (X->n > A->n) {