patch 9.2.0725: [security]: Stack out-of-bounds write in spell_soundfold_sal()

Problem:  [security]: A crafted spell file with non-collapsing SAL rules
          can make soundfold() write one byte past the end of the
          MAXWLEN result buffer.  This is the same class of
          out-of-bounds write as GHSA-q8mh-6qm3-25g4 (fixed in 9.2.0698
          for the SOFO branch), found while auditing the surrounding
          code.
Solution: Bound the single-byte SAL result writes and the terminating
          NUL to MAXWLEN - 1, matching the SOFO branch.

The single-byte branch of spell_soundfold_sal() guarded its writes with
"reslen < MAXWLEN", allowing reslen to reach MAXWLEN (254).  The trailing
"res[reslen] = NUL" then wrote at index 254 of the 254-byte stack buffer
res[MAXWLEN], an off-by-one out-of-bounds write.  Input is case-folded to
about 253 characters, so a 253-character argument together with a SAL map
that does not collapse (collapse_result false) reaches the boundary.

Related to previous issue
[GHSA-q8mh-6qm3-25g4](GHSA-q8mh-6qm3-25g4)
(9.2.0698)

Github Security Advisory:
GHSA-m3hf-xcm3-xhm2

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: 2 people

src/spell.c

@@ -3513,7 +3513,7 @@ spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
 			// no '<' rule used
 			i += k - 1;
 			z = 0;
-			while (*s != NUL && s[1] != NUL && reslen < MAXWLEN)
+			while (*s != NUL && s[1] != NUL && reslen < MAXWLEN - 1)
 			{
 			    if (reslen == 0 || res[reslen - 1] != *s)
 				res[reslen++] = *s;
@@ -3523,7 +3523,7 @@ spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
 			c = *s;
 			if (strstr((char *)pf, "^^") != NULL)
 			{
-			    if (c != NUL)
+			    if (c != NUL && reslen < MAXWLEN - 1)
 				res[reslen++] = c;
 			    STRMOVE(word, word + i + 1);
 			    i = 0;
@@ -3542,7 +3542,7 @@ spell_soundfold_sal(slang_T *slang, char_u *inword, char_u *res)
 
 	if (z0 == 0)
 	{
-	    if (k && !p0 && reslen < MAXWLEN && c != NUL
+	    if (k && !p0 && reslen < MAXWLEN - 1 && c != NUL
 		    && (!slang->sl_collapse || reslen == 0
 						     || res[reslen - 1] != c))
 		// condense only double letters

src/testdir/test_spellfile.vim

@@ -405,6 +405,30 @@ func Test_spellfile_format_error()
   let &rtp = save_rtp
 endfunc
 
+" An over-length soundfold() argument must not overflow the MAXWLEN result
+" buffer in the single-byte branch of spell_soundfold_sal().
+func Test_spellfile_soundfold_sal_overflow()
+  let save_enc = &encoding
+  set encoding=latin1
+  " A SAL map that appends without collapsing, so the result is not shorter
+  " than the input.
+  call writefile(['SET ISO8859-1', 'SAL collapse_result false',
+	\ 'SAL a aaaa', 'SAL b bbbb'], 'Xsal.aff')
+  call writefile(['2', 'hello', 'world'], 'Xsal.dic')
+  mkspell! Xsal Xsal
+  set spl=Xsal.latin1.spl spell
+
+  " 253 input characters hit the buffer boundary; the result must not exceed
+  " MAXWLEN - 1.
+  call assert_true(strlen(soundfold(repeat('a', 253))) <= 253)
+
+  set nospell spl& spelllang&
+  call delete('Xsal.aff')
+  call delete('Xsal.dic')
+  call delete('Xsal.latin1.spl')
+  let &encoding = save_enc
+endfunc
+
 " Test for format errors in suggest file
 func Test_sugfile_format_error()
   let save_rtp = &rtp

src/version.c

@@ -759,6 +759,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    725,
 /**/
     724,
 /**/