vim-patch:9.2.0846: [security]: heap buffer overflow in set_sofo()

Problem:  [security]: heap buffer overflow in set_sofo()
          (Yazan Balawneh)
Solution: Reset sl_sal_first (Yasuhiro Matsumoto).

A crafted spell file with an empty SN_SAL section before an SN_SOFO
section reaches set_sofo() with sl_sal_first[] already set to -1 by
set_sal_first(). The counting loop then under-counts colliding
multi-byte "from" characters, allocates an undersized list and writes
past its end.

Github Security Advisory:
https://github.com/vim/vim/security/advisories/GHSA-9jqx-hgpr-6v64

05c41c9223

Co-authored-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>

src/nvim/spellfile.c

@@ -1553,7 +1553,9 @@ static int set_sofo(slang_T *lp, const char *from, const char *to)
   lp->sl_sofo = true;
 
   // First count the number of items for each list.  Temporarily use
-  // sl_sal_first[] for this.
+  // sl_sal_first[] for this.  Reset it first: a preceding SN_SAL section
+  // may have set the entries to -1 via set_sal_first().
+  memset(lp->sl_sal_first, 0, sizeof(salfirst_T) * 256);
   for (p = from, s = to; *p != NUL && *s != NUL;) {
     const int c = mb_cptr2char_adv(&p);
     s += utf_ptr2len(s);

test/old/testdir/test_spellfile.vim

@@ -322,6 +322,11 @@ func Test_spellfile_format_error()
   " SN_SOFO: multi-byte characters in sofofrom and sofoto
   call Spellfile_Test(0z0600000000080002CF810002CF82FF000000000000000000000000, '')
 
+  " SN_SAL (empty) followed by SN_SOFO with two multi-byte 'from' characters
+  " sharing the same low byte.  A preceding SN_SAL poisons sl_sal_first[], so
+  " without a reset set_sofo() under-counts and writes out of bounds.
+  call Spellfile_Test(0z05000000000300000006000000000A0004CAABCEAB00024142FF000000000000000000000000, '')
+
   " SN_COMPOUND: compmax is less than 2
   call Spellfile_Test(0z08000000000101, 'E759:')