[2.31 COMMITTED] Fix use-after-free in glob when expanding ~user (bug 25414)

[Patsy Griffin <patsy@redhat.com>]

From: Andreas Schwab <schwab@suse.de>

The value of `end_name' points into the value of `dirname', thus don't
deallocate the latter before the last use of the former.

(cherry picked from commit ddc650e9b3dc916eab417ce9f79e67337b05035c)
---
 posix/glob.c | 25 +++++++++++++------------
 1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/posix/glob.c b/posix/glob.c
index cba9cd1819..4580cefb9f 100644
--- a/posix/glob.c
+++ b/posix/glob.c
@@ -827,31 +827,32 @@ __glob (const char *pattern, int flags, int
(*errfunc) (const char *, int),
              {
                size_t home_len = strlen (p->pw_dir);
                size_t rest_len = end_name == NULL ? 0 : strlen (end_name);
-               char *d;
+               char *d, *newp;
+               bool use_alloca = glob_use_alloca (alloca_used,
+                                                  home_len + rest_len + 1);

-               if (__glibc_unlikely (malloc_dirname))
-                 free (dirname);
-               malloc_dirname = 0;
-
-               if (glob_use_alloca (alloca_used, home_len + rest_len + 1))
-                 dirname = alloca_account (home_len + rest_len + 1,
-                                           alloca_used);
+               if (use_alloca)
+                 newp = alloca_account (home_len + rest_len + 1,
alloca_used);
                else
                  {
-                   dirname = malloc (home_len + rest_len + 1);
-                   if (dirname == NULL)
+                   newp = malloc (home_len + rest_len + 1);
+                   if (newp == NULL)
                      {
                        scratch_buffer_free (&pwtmpbuf);
                        retval = GLOB_NOSPACE;
                        goto out;
                      }
-                   malloc_dirname = 1;
                  }
-               d = mempcpy (dirname, p->pw_dir, home_len);
+               d = mempcpy (newp, p->pw_dir, home_len);
                if (end_name != NULL)
                  d = mempcpy (d, end_name, rest_len);
                *d = '\0';

+               if (__glibc_unlikely (malloc_dirname)
+                 free (dirname);
+               dirname = newp;
+               malloc_dirname = !use_alloca;
+
                dirlen = home_len + rest_len;
                dirname_modified = 1;
              }
--
2.21.1