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[Backport 2.28] mbedtls_mpi_sub_abs: Skip memcpy when redundant (#6701). #7001

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Feb 1, 2023
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[Backport 2.28] mbedtls_mpi_sub_abs: Skip memcpy when redundant (#6701). #7001

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4 changes: 4 additions & 0 deletions ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1,4 @@
Bugfix
* Fix potential undefined behavior in mbedtls_mpi_sub_abs(). Reported by
Pascal Cuoq using TrustInSoft Analyzer in #6701; observed independently by
Aaron Ucko under Valgrind.
2 changes: 1 addition & 1 deletion library/bignum.c
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Expand Up @@ -1280,7 +1280,7 @@ 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) {
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