neovim

commit 4263ec21c238b2a3da7b1ea711c415827bf63042
parent c74192788314cd34b86c9094b62490cfc7aad10e
Author: zeertzjq <zeertzjq@outlook.com>
Date:   Wed, 27 Aug 2025 10:16:59 +0800

vim-patch:9.1.1688: potential buffer overrun in bufwrite.c (#35497)

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: vim/vim#18095

https://github.com/vim/vim/commit/a19b019b8745f1c91e42c2b9440bddebfa67a17a

Co-authored-by: John Marriott <basilisk@internode.on.net>
Diffstat:
M
src/nvim/bufwrite.c
 | 
17
+++++++++--------

1 file changed, 9 insertions(+), 8 deletions(-)
diff --git a/src/nvim/bufwrite.c b/src/nvim/bufwrite.c
@@ -738,23 +738,24 @@ static int buf_write_make_backup(char *fname, bool append, FileInfo *file_info_o
       // the ones from the original file.
       // First find a file name that doesn't exist yet (use some
       // arbitrary numbers).
-      xstrlcpy(IObuff, fname, IOSIZE);
+      size_t dirlen = (size_t)(path_tail(fname) - fname);
+      assert(dirlen < MAXPATHL);
+      char tmp_fname[MAXPATHL];
+      xmemcpyz(tmp_fname, fname, dirlen);
       for (int i = 4913;; i += 123) {
-        char *tail = path_tail(IObuff);
-        size_t size = (size_t)(tail - IObuff);
-        snprintf(tail, IOSIZE - size, "%d", i);
-        if (!os_fileinfo_link(IObuff, &file_info)) {
+        snprintf(tmp_fname + dirlen, sizeof(tmp_fname) - dirlen, "%d", i);
+        if (!os_fileinfo_link(tmp_fname, &file_info)) {
           break;
         }
       }
-      int fd = os_open(IObuff,
+      int fd = os_open(tmp_fname,
                        O_CREAT|O_WRONLY|O_EXCL|O_NOFOLLOW, perm);
       if (fd < 0) {           // can't write in directory
         *backup_copyp = true;
       } else {
 #ifdef UNIX
         os_fchown(fd, (uv_uid_t)file_info_old->stat.st_uid, (uv_gid_t)file_info_old->stat.st_gid);
-        if (!os_fileinfo(IObuff, &file_info)
+        if (!os_fileinfo(tmp_fname, &file_info)
             || file_info.stat.st_uid != file_info_old->stat.st_uid
             || file_info.stat.st_gid != file_info_old->stat.st_gid
             || (int)file_info.stat.st_mode != perm) {
@@ -764,7 +765,7 @@ static int buf_write_make_backup(char *fname, bool append, FileInfo *file_info_o
         // Close the file before removing it, on MS-Windows we
         // can't delete an open file.
         close(fd);
-        os_remove(IObuff);
+        os_remove(tmp_fname);
       }
     }
   }