Simplify mbedtls_debug_print_mpi and fix the case of empty bignums
Rewrite mbedtls_debug_print_mpi to be simpler and smaller. Leverage
mbedtls_mpi_bitlen() instead of manually looking for the leading
zeros.
Fix #4608: the old code made an invalid memory dereference when
X->n==0 (freshly initialized bignum with the value 0).
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/mbedtls_debug_print_mpi.txt b/ChangeLog.d/mbedtls_debug_print_mpi.txt
new file mode 100644
index 0000000..d1b4f5b
--- /dev/null
+++ b/ChangeLog.d/mbedtls_debug_print_mpi.txt
@@ -0,0 +1,5 @@
+Bugfix
+ * Fix a crash in mbedtls_mpi_debug_mpi on a bignum having 0 limbs. This
+ could notably be triggered by setting the TLS debug level to 3 or above
+ and using a Montgomery curve for the key exchange. Reported by lhuang04
+ in #4578. Fixes #4608.