patch 9.2.0075: [security]: Buffer underflow with emacs tag file

Problem:  When parsing a malformed Emacs-style tags file, a 1-byte
          heap-buffer-underflow read occurs if the 0x7f delimiter
          appears at the very beginning of a line. This happens
          because the code attempts to scan backward for a tag
          name from the delimiter without checking if space exists.
          (ehdgks0627, un3xploitable)
Solution: Add a check to ensure the delimiter (p_7f) is not at the
          start of the buffer (lbuf) before attempting to isolate
          the tag name.

GitHub Advisory:
https://github.com/vim/vim/security/advisories/GHSA-xcc8-r6c5-hvwv

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

src/tag.c

@@ -1901,6 +1901,9 @@ emacs_tags_new_filename(findtags_state_T *st)
 
     for (p = st->ebuf; *p && *p != ','; p++)
 	;
+    // invalid
+    if (*p == NUL)
+	return;
     *p = NUL;
 
     // check for an included tags file.
@@ -2019,6 +2022,9 @@ etag_fail:
     }
     else			    // second format: isolate tagname
     {
+	if (p_7f == lbuf)
+	    goto etag_fail;
+
 	// find end of tagname
 	for (p = p_7f - 1; !vim_iswordc(*p); --p)
 	    if (p == lbuf)

src/testdir/test_taglist.vim

@@ -301,4 +301,35 @@ func Test_tag_complete_with_overlong_line()
   set tags&
 endfunc
 
+" This used to crash Vim
+func Test_evil_emacs_tagfile()
+  CheckFeature emacs_tags
+  let longline = repeat('a', 515)
+  call writefile([
+	\ "\x0c",
+	\ longline
+	\ ], 'Xtags', 'D')
+  set tags=Xtags
+
+  call assert_fails(':tag a', 'E426:')
+
+  set tags&
+endfunc
+
+" This used to crash Vim due to a heap-buffer-underflow
+func Test_emacs_tagfile_underflow()
+  CheckFeature emacs_tags
+  " The sequence from the crash artifact:
+  let lines = [
+    \ "\x0c\xff\xffT\x19\x8a",
+    \ "\x19\x19\x0dtags\x19\x19\x19\x00\xff\xff\xff",
+    \ "\x7f3\x0c"
+    \ ]
+  call writefile(lines, 'Xtags', 'D')
+  set tags=Xtags
+  call assert_fails(':tag a', 'E431:')
+
+  set tags&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -734,6 +734,10 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    75,
+/**/
+    74,
 /**/
     73,
 /**/