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From: Aleksa Sarai <cyphar@cyphar.com>
To: Jann Horn <jannh@google.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
	Jeff Layton <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	Arnd Bergmann <arnd@arndb.de>,
	David Howells <dhowells@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Christian Brauner <christian@brauner.io>,
	Andy Lutomirski <luto@kernel.org>,
	Eric Biederman <ebiederm@xmission.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Tycho Andersen <tycho@tycho.ws>,
	David Drysdale <drysdale@google.com>,
	Chanho Min <chanho.min@lge.com>, Oleg Nesterov <oleg@redhat.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>, Namhyung Kim <namhyung@kernel.org>,
	Aleksa Sarai <asarai@suse.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
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	linux-s390 <linux-s390@vger.kernel.org>,
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	sparclinux@vger.kernel.org
Subject: Re: [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions
Date: Wed, 18 Sep 2019 17:46:15 +0200
Message-ID: <20190918154615.suruy5v5xjftfwyl@yavin.microfocus.com> (raw)
In-Reply-To: <20190918135100.sdxdmdluq6wlwryv@yavin.microfocus.com>

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On 2019-09-18, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2019-09-17, Jann Horn <jannh@google.com> wrote:
> > On Wed, Sep 4, 2019 at 10:21 PM Aleksa Sarai <cyphar@cyphar.com> wrote:
> > > The ability for userspace to "re-open" file descriptors through
> > > /proc/self/fd has been a very useful tool for all sorts of usecases
> > > (container runtimes are one common example). However, the current
> > > interface for doing this has resulted in some pretty subtle security
> > > holes. Userspace can re-open a file descriptor with more permissions
> > > than the original, which can result in cases such as /proc/$pid/exe
> > > being re-opened O_RDWR at a later date even though (by definition)
> > > /proc/$pid/exe cannot be opened for writing. When combined with O_PATH
> > > the results can get even more confusing.
> > [...]
> > > Instead we have to restrict it in such a way that it doesn't break
> > > (good) users but does block potential attackers. The solution applied in
> > > this patch is to restrict *re-opening* (not resolution through)
> > > magic-links by requiring that mode of the link be obeyed. Normal
> > > symlinks have modes of a+rwx but magic-links have other modes. These
> > > magic-link modes were historically ignored during path resolution, but
> > > they've now been re-purposed for more useful ends.
> > 
> > Thanks for dealing with this issue!
> > 
> > [...]
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index 209c51a5226c..54d57dad0f91 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -872,7 +872,7 @@ void nd_jump_link(struct path *path)
> > >
> > >         nd->path = *path;
> > >         nd->inode = nd->path.dentry->d_inode;
> > > -       nd->flags |= LOOKUP_JUMPED;
> > > +       nd->flags |= LOOKUP_JUMPED | LOOKUP_MAGICLINK_JUMPED;
> > >  }
> > [...]
> > > +static int trailing_magiclink(struct nameidata *nd, int acc_mode,
> > > +                             fmode_t *opath_mask)
> > > +{
> > > +       struct inode *inode = nd->link_inode;
> > > +       fmode_t upgrade_mask = 0;
> > > +
> > > +       /* Was the trailing_symlink() a magic-link? */
> > > +       if (!(nd->flags & LOOKUP_MAGICLINK_JUMPED))
> > > +               return 0;
> > > +
> > > +       /*
> > > +        * Figure out the upgrade-mask of the link_inode. Since these aren't
> > > +        * strictly POSIX semantics we don't do an acl_permission_check() here,
> > > +        * so we only care that at least one bit is set for each upgrade-mode.
> > > +        */
> > > +       if (inode->i_mode & S_IRUGO)
> > > +               upgrade_mask |= FMODE_PATH_READ;
> > > +       if (inode->i_mode & S_IWUGO)
> > > +               upgrade_mask |= FMODE_PATH_WRITE;
> > > +       /* Restrict the O_PATH upgrade-mask of the caller. */
> > > +       if (opath_mask)
> > > +               *opath_mask &= upgrade_mask;
> > > +       return may_open_magiclink(upgrade_mask, acc_mode);
> > >  }
> > 
> > This looks racy because entries in the file descriptor table can be
> > switched out as long as task->files->file_lock isn't held. Unless I'm
> > missing something, something like the following (untested) would
> > bypass this restriction:
> 
> You're absolutely right -- good catch!
> 
> > Perhaps you could change nd_jump_link() to "void nd_jump_link(struct
> > path *path, umode_t link_mode)", and let proc_pid_get_link() pass the
> > link_mode through from an out-argument of .proc_get_link()? Then
> > proc_fd_link() could grab the proper mode in a race-free manner. And
> > nd_jump_link() could stash the mode in the nameidata.
> 
> This indeed does appear to be the simplest solution -- I'm currently
> testing a variation of the patch you proposed (with a few extra bits to
> deal with nd_jump_link and proc_get_link being used elsewhere).
> 
> I'll include this change (assuming it fixes the flaw you found) in the
> v13 series I'll send around next week. Thanks, Jann!

In case you're interested -- I've also included a selftest based on this
attack in my series (though it uses CLONE_FILES so that we could also
test O_EMPTYPATH, which wasn't affected because it didn't go through
procfs and thus couldn't hit the "outdated inode->i_mode" problem).

The attack script succeeds around 20% of the time on the original
patchset, and with the updated patchset it doesn't succeed in several
hundred thousand attempts (which I've repeated a few times).

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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  reply index

Thread overview: 65+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-04 20:19 [PATCH v12 00/12] namei: openat2(2) path resolution restrictions Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Aleksa Sarai
2019-09-04 20:48   ` Linus Torvalds
2019-09-04 21:00   ` Randy Dunlap
2019-09-05  7:32   ` Peter Zijlstra
2019-09-05  9:26     ` Aleksa Sarai
2019-09-05  9:43       ` Peter Zijlstra
2019-09-05 10:57         ` Peter Zijlstra
2019-09-11 10:37           ` Aleksa Sarai
2019-09-05 13:35         ` Aleksa Sarai
2019-09-05 17:01         ` Aleksa Sarai
2019-09-05  8:43   ` Rasmus Villemoes
2019-09-05  9:50     ` Aleksa Sarai
2019-09-05 10:45       ` Christian Brauner
2019-09-05  9:09   ` Andreas Schwab
2019-09-05 10:13     ` [PATCH v12 01/12] lib: introduce copy_struct_{to, from}_user helpers Gabriel Paubert
2019-09-05 11:05   ` [PATCH v12 01/12] lib: introduce copy_struct_{to,from}_user helpers Christian Brauner
2019-09-05 11:17     ` Rasmus Villemoes
2019-09-05 11:29       ` Christian Brauner
2019-09-05 13:40     ` Aleksa Sarai
2019-09-05 11:09   ` Christian Brauner
2019-09-05 11:27     ` Aleksa Sarai
2019-09-05 11:40       ` Christian Brauner
2019-09-05 18:07   ` Al Viro
2019-09-05 18:23     ` Christian Brauner
2019-09-05 18:28       ` Al Viro
2019-09-05 18:35         ` Christian Brauner
2019-09-05 19:56         ` Aleksa Sarai
2019-09-05 22:31           ` Al Viro
2019-09-06  7:00           ` Christian Brauner
2019-09-05 23:00     ` Aleksa Sarai
2019-09-05 23:49       ` Al Viro
2019-09-06  0:09         ` Aleksa Sarai
2019-09-06  0:14         ` Al Viro
2019-09-04 20:19 ` [PATCH v12 02/12] clone3: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 03/12] sched_setattr: switch to copy_struct_{to,from}_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 04/12] perf_event_open: switch to copy_struct_from_user() Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 05/12] namei: obey trailing magic-link DAC permissions Aleksa Sarai
2019-09-17 21:30   ` Jann Horn
2019-09-18 13:51     ` Aleksa Sarai
2019-09-18 15:46       ` Aleksa Sarai [this message]
2019-09-04 20:19 ` [PATCH v12 06/12] procfs: switch magic-link modes to be more sane Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 07/12] open: O_EMPTYPATH: procfs-less file descriptor re-opening Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 08/12] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 09/12] namei: LOOKUP_IN_ROOT: chroot-like path resolution Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 10/12] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai
2019-09-04 21:09   ` Linus Torvalds
2019-09-04 21:35     ` Linus Torvalds
2019-09-04 21:36       ` Linus Torvalds
2019-09-04 21:48     ` Aleksa Sarai
2019-09-04 22:16       ` Linus Torvalds
2019-09-04 22:31       ` David Howells
2019-09-04 22:38         ` Linus Torvalds
2019-09-04 23:29           ` Al Viro
2019-09-04 23:44             ` Linus Torvalds
2019-09-04 20:19 ` [PATCH v12 11/12] open: openat2(2) syscall Aleksa Sarai
2019-09-04 21:00   ` Randy Dunlap
2019-09-07 12:40   ` Jeff Layton
2019-09-07 16:58     ` Linus Torvalds
2019-09-07 17:42       ` Andy Lutomirski
2019-09-07 17:45         ` Linus Torvalds
2019-09-07 18:15           ` Andy Lutomirski
2019-09-10  6:35           ` Ingo Molnar
2019-09-08 16:24     ` Aleksa Sarai
2019-09-04 20:19 ` [PATCH v12 12/12] selftests: add openat2(2) selftests Aleksa Sarai

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