From e2bd8600b873d2cd1f9d667c28cba8b1dba18839 Mon Sep 17 00:00:00 2001
From: Bram Moolenaar <Bram@vim.org>
Date: Wed, 18 May 2022 13:11:57 +0100
Subject: [PATCH] patch 8.2.4977: memory access error when substitute
 expression changes window

Problem:    Memory access error when substitute expression changes window.
Solution:   Disallow changing window in substitute expression.
---
 src/ex_cmds.c                   | 11 +++++++++++
 src/testdir/test_substitute.vim | 14 ++++++++++++++
 2 files changed, 25 insertions(+)

diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index aa97b40..0a22f59 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -4289,12 +4289,17 @@ do_sub(exarg_T *eap)
 		// Save flags for recursion.  They can change for e.g.
 		// :s/^/\=execute("s#^##gn")
 		subflags_save = subflags;
+
+                // Disallow changing text or switching window in an expression.
+                ++textwinlock;
 #endif
 		// get length of substitution part
 		sublen = vim_regsub_multi(&regmatch,
 				    sub_firstlnum - regmatch.startpos[0].lnum,
 				    sub, sub_firstline, FALSE, p_magic, TRUE);
 #ifdef FEAT_EVAL
+                --textwinlock;
+
 		// If getting the substitute string caused an error, don't do
 		// the replacement.
 		// Don't keep flags set by a recursive call.
@@ -4395,9 +4400,15 @@ do_sub(exarg_T *eap)
 		mch_memmove(new_end, sub_firstline + copycol, (size_t)copy_len);
 		new_end += copy_len;
 
+#ifdef FEAT_EVAL
+                ++textwinlock;
+#endif
 		(void)vim_regsub_multi(&regmatch,
 				    sub_firstlnum - regmatch.startpos[0].lnum,
 					   sub, new_end, TRUE, p_magic, TRUE);
+#ifdef FEAT_EVAL
+                --textwinlock;
+#endif
 		sub_nsubs++;
 		did_sub = TRUE;
 
diff --git a/src/testdir/test_substitute.vim b/src/testdir/test_substitute.vim
index 3e6bc5c..bda96f6 100644
--- a/src/testdir/test_substitute.vim
+++ b/src/testdir/test_substitute.vim
@@ -761,3 +761,17 @@ func Test_using_old_sub()
   bwipe!
   set nocompatible
 endfunc
+
+" This was switching windows in between computing the length and using it.
+func Test_sub_change_window()
+  silent! lfile
+  sil! norm o0000000000000000000000000000000000000000000000000000
+  func Repl()
+    lopen
+  endfunc
+  silent!  s/\%')/\=Repl()
+  bwipe!
+  bwipe!
+  delfunc Repl
+endfunc
+
-- 
1.8.3.1