tomoyo: fallback to realpath if symlink's pathname does not exist

Alfred Agrell found that TOMOYO cannot handle execveat(AT_EMPTY_PATH)
inside chroot environment where /dev and /proc are not mounted, for
commit 51f39a1f ("syscalls: implement execveat() system call") missed
that TOMOYO tries to canonicalize argv[0] when the filename fed to the
executed program as argv[0] is supplied using potentially nonexistent
pathname.

Since "/dev/fd/<fd>" already lost symlink information used for obtaining
that <fd>, it is too late to reconstruct symlink's pathname. Although
<filename> part of "/dev/fd/<fd>/<filename>" might not be canonicalized,
TOMOYO cannot use tomoyo_realpath_nofollow() when /dev or /proc is not
mounted. Therefore, fallback to tomoyo_realpath_from_path() when
tomoyo_realpath_nofollow() failed.
Reported-by: Alfred Agrell <blubban@gmail.com>
Closes: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1082001
Fixes: 51f39a1f ("syscalls: implement execveat() system call")
Cc: stable@vger.kernel.org # v3.19+
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

security/tomoyo/domain.c

@@ -723,10 +723,13 @@ int tomoyo_find_next_domain(struct linux_binprm *bprm)
 	ee->r.obj = &ee->obj;
 	ee->obj.path1 = bprm->file->f_path;
 	/* Get symlink's pathname of program. */
-	retval = -ENOENT;
 	exename.name = tomoyo_realpath_nofollow(original_name);
-	if (!exename.name)
-		goto out;
+	if (!exename.name) {
+		/* Fallback to realpath if symlink's pathname does not exist. */
+		exename.name = tomoyo_realpath_from_path(&bprm->file->f_path);
+		if (!exename.name)
+			goto out;
+	}
 	tomoyo_fill_path_info(&exename);
 retry:
 	/* Check 'aggregator' directive. */