Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Mon, 16 Jul 2012 09:55:51 +0200
From: Sebastian Krahmer <krahmer@...e.de>
To: oss-security@...ts.openwall.com
Cc: Tyler Hicks <tyhicks@...onical.com>,
	Kurt Seifried <kseifried@...hat.com>,
	Marcus Meissner <meissner@...e.de>,
	Dan Rosenberg <dan.j.rosenberg@...il.com>
Subject: Re: Re: ecryptfs headsup


Indeed, there are various issues in the ecryptfs tool that made me scratch
my head but are not serious vulnerabilities and cannot be fixed so easily
w/o rewriting the entire file handling. Yes, some of the checks could
probably be removed.
So the main focus was that its runs with euid of the user. :)
The one real issue was the missing MS_NOSUID|MS_NODEV.

Sebastian

On Sat, Jul 14, 2012 at 07:41:44AM +0200, Jason A. Donenfeld wrote:
> Hi,
> 
> There's another almost-hole (almost, like the umount env clearing)
> that the patch in that bug report fixes:
> 
> At line 538, this code is added:
> 
> 	if (strstr(alias, "..")) {
> 		fputs("Invalid alias", stderr);
> 		exit(1);
> 	}
> 
> Ostensibly this is to prevent the subsequent call to
> 
>         lock_counter(pwd->pw_name, uid, alias);
> 
> from crafting the path name like this:
> 
>        asprintf(&f, "%s/%s-%s-%s", TMP, FSTYPE, u, alias)
> 
> where alias might have ".." in it, and then making a file like this:
> 
>       open(f, O_RDWR | O_CREAT | O_NOFOLLOW, 0600)
> 
> which takes special care to specify O_NOFOLLOW too, and then
> afterwards, it checks to see if it's owned by the correct user and
> makes certain it's a regular file.
> 
> Fortunately, the call at the beginning of the program to seteuid(uid)
> prevents this from being too traumatic. But, the strstr check was
> added apparently as a security consideration; correct me if I'm
> mistaken.
> 
> 
> 
> 
> There's actually one use of the alias variable before such sanitation
> that is almost a big hole, but isn't, due again to the seteuid(uid) at
> the beginning of the program. At line 525 we have:
> 
> if (read_config(pwd->pw_dir, uid, alias, &src, &dest, &opts2) < 0) {
> 
> read_config forms a file name like this:
> 
> if (asprintf(&fnam, "%s/.ecryptfs/%s.conf", pw_dir, alias) < 0) {
> 
> It then makes a smart security check to ensure that it's not a symlink
> and that it's owned by uid:
> 
> if (stat(fnam, &mstat)!=0 || (!S_ISREG(mstat.st_mode) || mstat.st_uid!=uid)) {
>     fputs("Bad file\n", stderr);
>     free(fnam);
>     return -1;
> }
> 
> After it does this, it opens the file for reading with a standard fopen:
> 
> fin = fopen(fnam, "r");
> 
> And then it reads the various settings out of fin.
> 
> There's a race condition on fnam, since you could make fnam a valid
> file, but immediately after the stat(2) call, swap it for a symlink.
> I'm not sure this is really an issue, though, since geteuid(uid) has
> already been called, so it's not as if the subsequent fopen is allowed
> to read just any file. It does pose the question, though, of why that
> stat check is there in the first place, if it can be subverted like
> that. The solution is just to open the fd first, and then check the
> permissions and whatnot with fstat(2).
> 
> Jason Donenfeld

-- 

~ perl self.pl
~ $_='print"\$_=\47$_\47;eval"';eval
~ krahmer@...e.de - SuSE Security Team

---
SUSE LINUX Products GmbH,
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer, HRB 16746 (AG Nürnberg)
Maxfeldstraße 5
90409 Nürnberg
Germany

Powered by blists - more mailing lists

Your e-mail address:

Please check out the Open Source Software Security Wiki, which is counterpart to this mailing list.

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.