Fix use-after-free in glob when expanding ~user (bug 25414)

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 with
 changes from commit d711a00f93fa964f41a53839228598fbf1a6b482)

2 files changed, 21 insertions, 11 deletions

diff --git a/NEWS b/NEWS
index cb62587dbb..b18a0f3d29 100644
--- a/NEWS
+++ b/NEWS
@@ -394,6 +394,9 @@ Security related changes:
 * A use-after-free vulnerability in clntudp_call in the Sun RPC system has been
   fixed (CVE-2017-12133).
 
+* A use-after-free vulnerability in the glob function when expanding ~user has
+  been fixed (CVE-2020-1752).
+
 The following bugs are resolved with this release:
 
   [984] network: Respond to changed resolv.conf in gethostbyname
@@ -621,6 +624,7 @@ The following bugs are resolved with this release:
   [21839] localedata: Fix LC_MONETARY for ta_LK
   [21844] localedata: Fix Latin characters and Months Sequence.
   [21848] localedata: Fix mai_NP Title Name
+  [25414] 'glob' use-after-free bug (CVE-2020-1752)
 
 
 Version 2.25
diff --git a/posix/glob.c b/posix/glob.c
index b2273ea7bc..0c6eeb3637 100644
--- a/posix/glob.c
+++ b/posix/glob.c
@@ -947,26 +947,32 @@ 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);
 
-		if (__glibc_unlikely (malloc_dirname))
-		  free (dirname);
-		malloc_dirname = 0;
+		char *d, *newp;
+		bool use_alloca = glob_use_alloca (alloca_used,
+						   home_len + rest_len + 1);
 
-		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)
 		      {
 			free (malloc_pwtmpbuf);
 			retval = GLOB_NOSPACE;
 			goto out;
 		      }
-		    malloc_dirname = 1;
 		  }
-		*((char *) mempcpy (mempcpy (dirname, p->pw_dir, home_len),
-				    end_name, rest_len)) = '\0';
+
+		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;