Changelog for the fix to mbedtls_mpi_exp_mod(A=0)
In Mbed TLS 2.26.0, the bug was hard to trigger, since all methods for
parsing a bignum (mbedtls_mpi_read_xxx functions) constructed an mbedtls_mpi
object with at least one limb.
In the development branch, after the commit
"New internal function mbedtls_mpi_resize_clear", this bug could be
triggered by a TLS server, by passing invalid custom Diffie-Hellman
parameters with G=0 transmitted as a 0-length byte string.
Since the behavior change in mbedtls_mpi_read_binary and
mbedtls_mpi_read_binary_le (constructing 0 limbs instead of 1 when passed
empty input) turned out to have consequences despite being in principle an
internal detail, mention it in the changelog.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/ChangeLog.d/mpi_exp_mod-zero.txt b/ChangeLog.d/mpi_exp_mod-zero.txt
new file mode 100644
index 0000000..9df9031
--- /dev/null
+++ b/ChangeLog.d/mpi_exp_mod-zero.txt
@@ -0,0 +1,7 @@
+Bugfix
+ * Fix a null pointer dereference when mbedtls_mpi_exp_mod() was called with
+ A=0 represented with 0 limbs. Up to and including Mbed TLS 2.26, this bug
+ could not be triggered by code that constructed A with one of the
+ mbedtls_mpi_read_xxx functions (including in particular TLS code) since
+ those always built an mpi object with at least one limb.
+ Credit to OSS-Fuzz. Fixes #4641.