hush: in some cases, expand_on_ifs() relied of uninitialized memory

The n > 0 check to prevent access to the last byte of non-existing argv[-1]
wasn't enough. Switched to making sure there are initialized (zero) bytes there.

A predictable testcase is rather hard to construct, unfortunately,
contents of memory depends on allocator behavior and whatnot.

function                                             old     new   delta
o_save_ptr_helper                                    119     137     +18
expand_on_ifs                                        345     339      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/1 up/down: 18/-6)              Total: 12 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 13 insertions, 2 deletions

diff --git a/shell/hush.c b/shell/hush.c
index 02fb1b5ef..1ac2db381 100644
--- a/shell/hush.c
+++ b/shell/hush.c
@@ -3069,6 +3069,13 @@ static int o_save_ptr_helper(o_string *o, int n)
                         o->data = xrealloc(o->data, o->maxlen + 1);
                         list = (char**)o->data;
                         memmove(list + n + 0x10, list + n, string_len);
+                        /*
+                         * expand_on_ifs() has a "previous argv[] ends in IFS?"
+                         * check. (grep for -prev-ifs-check-).
+                         * Ensure that argv[-1][last] is not garbage
+                         * but zero bytes, to save index check there.
+                         */
+                        list[n + 0x10 - 1] = 0;
                         o->length += 0x10 * sizeof(list[0]);
                 } else {
                         debug_printf_list("list[%d]=%d string_start=%d\n",
@@ -5797,12 +5804,16 @@ static int expand_on_ifs(o_string *output, int n, const char *str)
                 /* Start new word... but not always! */
                 /* Case "v=' a'; echo ''$v": we do need to finalize empty word: */
                 if (output->has_quoted_part
-                /* Case "v=' a'; echo $v":
+                /*
+                 * Case "v=' a'; echo $v":
                  * here nothing precedes the space in $v expansion,
                  * therefore we should not finish the word
                  * (IOW: if there *is* word to finalize, only then do it):
+                 * It's okay if this accesses the byte before first argv[]:
+                 * past call to o_save_ptr() cleared it to zero byte
+                 * (grep for -prev-ifs-check-).
                  */
-                 || (n > 0 && output->data[output->length - 1])
+                 || output->data[output->length - 1]
                 ) {
  new_word:
                         o_addchr(output, '\0');