patch 9.0.2143: [security]: buffer-overflow in ex_substitute

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>

src/ex_cmds.c

@@ -4650,7 +4650,7 @@ ex_substitute(exarg_T *eap)
 		     * too many calls to alloc()/free()).
 		     */
 		    new_start_len = needed_len + 50;
-		    if ((new_start = alloc(new_start_len)) == NULL)
+		    if ((new_start = alloc_clear(new_start_len)) == NULL)
 			goto outofmem;
 		    *new_start = NUL;
 		    new_end = new_start;
@@ -4667,7 +4667,7 @@ ex_substitute(exarg_T *eap)
 		    if (needed_len > (int)new_start_len)
 		    {
 			new_start_len = needed_len + 50;
-			if ((p1 = alloc(new_start_len)) == NULL)
+			if ((p1 = alloc_clear(new_start_len)) == NULL)
 			{
 			    vim_free(new_start);
 			    goto outofmem;

src/testdir/test_crash.vim

@@ -117,7 +117,7 @@ func Test_crash1_2()
   " The following used to crash Vim
   let opts = #{cmd: 'sh'}
   let vim  = GetVimProg()
-  let result = 'X_crash1_1_result.txt'
+  let result = 'X_crash1_2_result.txt'
 
   let buf = RunVimInTerminal('sh', opts)
 
@@ -149,6 +149,17 @@ func Test_crash1_2()
     \ ' ; echo "crash 4: [OK]" >> '.. result .. "\<cr>")
   call TermWait(buf, 150)
 
+  let file = 'crash/poc_ex_substitute'
+  let cmn_args = "%s -u NONE -i NONE -n -e -s -S %s -c ':qa!'"
+  let args = printf(cmn_args, vim, file)
+  " just make sure it runs, we don't care about the resulting echo
+  call term_sendkeys(buf, args .. "\<cr>")
+  " There is no output generated in Github CI for the asan clang build.
+  " so just skip generating the ouput.
+  " call term_sendkeys(buf, args ..
+  "   \ ' &&  echo "crash 5: [OK]" >> '.. result .. "\<cr>")
+  call TermWait(buf, 150)
+
   " clean up
   exe buf .. "bw!"
 

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2143,
 /**/
     2142,
 /**/