patch 9.0.2143: [security]: buffer-overflow in ex_substitutev9.0.2143

Problem:  [security]: buffer-overflow in ex_substitute
Solution: clear memory after allocating

When allocating the new_start pointer in ex_substitute() the memory
pointer points to some garbage that the following for loop in
ex_cmds.c:4743 confuses and causes it to accessing the new_start pointer
beyond it's size, leading to a buffer-overlow.

So fix this by using alloc_clear() instead of alloc(), which will
clear the memory by NUL and therefore cause the loop to terminate
correctly.

Reported by @henices, thanks!

closes: #13596
Signed-off-by: Christian Brabandt <cb@256bit.org>

1 files changed, 0 insertions, 0 deletions

diff --git a/src/testdir/crash/poc_ex_substitute b/src/testdir/crash/poc_ex_substitute
new file mode 100644
index 0000000000..bcf1286512
--- /dev/null
+++ b/src/testdir/crash/poc_ex_substitute