patch 9.1.1688: potential buffer overrun in bufwrite.c

Problem:  potential buffer overrun in bufwrite.c
Solution: Use a temporary variable (John Marriott)

In my Windows 11 Pro 64-bit build MAXPATHL is 1024 and IOSIZE is 1025.
In my Archlinux Linux 64-bit build MAXPATHL is 4096 and IOSIZE is 1025.

In funuction buf_write():
There is a check (line 713) that makes sure the length of fname is less
than MAXPATHL. There is a call to STRCPY() (line 1208) which copies the
string at fname into IObuff (which has size IOSIZE). For Unix builds
fname is set to sfname which may or may not be shorter. However, if
sfname is NULL sfname is set to fname.

Therefore, in builds where MAXPATHL > IOSIZE (eg in my linux build), it
is theoretically possible for the STRCPY() call to exceed the bounds of
IObuff.

This PR addresses this by copying fname into a local variable that has
the same maximum size as fname.

In addition:
Given that the filename is unconditionally overwritten in the for loop,
only copy the directory portion of fname. Move variable i closer to
where it is used.

closes: #18095

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>

src/bufwrite.c

@@ -1173,8 +1173,6 @@ buf_write(
 #if defined(UNIX) || defined(MSWIN)
 	else if ((bkc & BKC_AUTO))	// "auto"
 	{
-	    int		i;
-
 # ifdef UNIX
 	    // Don't rename the file when:
 	    // - it's a hard link
@@ -1201,18 +1199,24 @@ buf_write(
 #  endif
 # endif
 	    {
+		size_t	dirlen;
+		char_u	tmp_fname[MAXPATHL];
+		int	i;
+
 		// Check if we can create a file and set the owner/group to
 		// the ones from the original file.
 		// First find a file name that doesn't exist yet (use some
 		// arbitrary numbers).
-		STRCPY(IObuff, fname);
+		dirlen = (size_t)(gettail(fname) - fname);
+		vim_strncpy(tmp_fname, fname, dirlen);
 		fd = -1;
 		for (i = 4913; ; i += 123)
 		{
-		    sprintf((char *)gettail(IObuff), "%d", i);
+		    vim_snprintf((char *)tmp_fname + dirlen,
-		    if (mch_lstat((char *)IObuff, &st) < 0)
+			sizeof(tmp_fname) - dirlen, "%d", i);
+		    if (mch_lstat((char *)tmp_fname, &st) < 0)
 		    {
-			fd = mch_open((char *)IObuff,
+			fd = mch_open((char *)tmp_fname,
 				    O_CREAT|O_WRONLY|O_EXCL|O_NOFOLLOW, perm);
 			if (fd < 0 && errno == EEXIST)
 			    // If the same file name is created by another
@@ -1230,7 +1234,7 @@ buf_write(
 #  ifdef HAVE_FCHOWN
 		    vim_ignored = fchown(fd, st_old.st_uid, st_old.st_gid);
 #  endif
-		    if (mch_stat((char *)IObuff, &st) < 0
+		    if (mch_stat((char *)tmp_fname, &st) < 0
 			    || st.st_uid != st_old.st_uid
 			    || st.st_gid != st_old.st_gid
 			    || (long)st.st_mode != perm)
@@ -1239,7 +1243,7 @@ buf_write(
 		    // Close the file before removing it, on MS-Windows we
 		    // can't delete an open file.
 		    close(fd);
-		    mch_remove(IObuff);
+		    mch_remove(tmp_fname);
 # ifdef MSWIN
 		    // MS-Windows may trigger a virus scanner to open the
 		    // file, we can't delete it then.  Keep trying for half a
@@ -1249,10 +1253,10 @@ buf_write(
 
 			for (try = 0; try < 10; ++try)
 			{
-			    if (mch_lstat((char *)IObuff, &st) < 0)
+			    if (mch_lstat((char *)tmp_fname, &st) < 0)
 				break;
 			    ui_delay(50L, TRUE);  // wait 50 msec
-			    mch_remove(IObuff);
+			    mch_remove(tmp_fname);
 			}
 		    }
 # endif

src/version.c

@@ -724,6 +724,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1688,
 /**/
     1687,
 /**/