From: Andy Lutomirski <luto@kernel.org>
To: Al Viro <viro@zeniv.linux.org.uk>
Cc: Jann Horn <jannh@google.com>,
Linux API <linux-api@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Linux FS Devel <linux-fsdevel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: new ...at() flag: AT_NO_JUMPS
Date: Thu, 4 May 2017 17:44:19 -0700 [thread overview]
Message-ID: <CALCETrXE2-841_VRgDJxRZBYAfvSe+GQN+0Jkfh3k1pL8XfDaA@mail.gmail.com> (raw)
In-Reply-To: <20170505003030.GM29622@ZenIV.linux.org.uk>
On Thu, May 4, 2017 at 5:30 PM, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Mon, May 01, 2017 at 07:36:52PM +0200, Jann Horn wrote:
>
>> Oh, nice!
>>
>> It looks like this is somewhat similar to the old O_BENEATH proposal,
>> but because the intentions behind the proposals are different
>> (application sandboxing versus permitting an application to restrict its
>> own filesystem accesses), the semantics differ: AT_NO_JUMPS
>> doesn't prevent starting the path with "/", but does prevent mountpoint
>> traversal. Is that correct?
>
> It prevents both, actually - I missed that in description, but this
> if (unlikely(nd->flags & LOOKUP_NO_JUMPS))
> return -ELOOP;
> in nd_jump_root() affects absolute pathnames same way as it affects
> absolute symlinks.
>
> It's not quite O_BENEATH, and IMO it's saner that way - a/b/c/../d is
> bloody well allowed, and so are relative symlinks that do not lead out of
> the subtree. If somebody has a good argument in favour of flat-out
> ban on .. (_other_ than "other guys do it that way, and it doesn't need
> to make sense 'cuz security!!1!!!", please), I'd be glad to hear it.
I don't have an argument for allowing '..'. I think it would be okay
to disallow it, but I don't think it matters all that much either way.
>
> As for mountpoint crossing... it might make sense to split those.
> O_BENEATH allowed it, and if we want AT_BENEATH to match that - let's
> do it. Then this one would become AT_BENEATH | AT_XDEV (the latter named
> after find(1) option, obviously).
>
> So how about this:
>
> AT_BENEATH:
> * no absolute pathnames
> * no absolute symlinks
> * no procfs-style symlinks
> * no traversal of .. when we are at the same place where we'd started
> (dir/../file is allowed, dir/../.. isn't)
>
> AT_XDEV:
> * no mountpoint crossing allowed
>
> For the latter I would prefer -EXDEV, for obvious reasons. For the former...
> not sure. I'm not too happy about -ELOOP, but -EPERM (as with O_BENEATH)
> is an atrocity - it's even more overloaded.
>
> Suggestions?
-EDOTDOT would be amusing.
next prev parent reply other threads:[~2017-05-05 0:45 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-29 22:04 new ...at() flag: AT_NO_JUMPS Al Viro
2017-04-29 23:17 ` Andy Lutomirski
2017-04-29 23:25 ` Al Viro
2017-04-30 1:13 ` Andy Lutomirski
2017-04-30 4:38 ` Matthew Wilcox
2017-04-30 16:10 ` Al Viro
2017-05-01 4:52 ` Andy Lutomirski
2017-05-01 5:15 ` Al Viro
2017-05-01 17:36 ` Jann Horn
2017-05-01 19:37 ` Andy Lutomirski
2017-05-05 0:30 ` Al Viro
2017-05-05 0:44 ` Andy Lutomirski [this message]
2017-05-05 1:06 ` Al Viro
2017-05-05 1:27 ` Linus Torvalds
2017-05-05 3:00 ` Al Viro
2017-05-05 4:01 ` Linus Torvalds
2017-05-05 4:31 ` Andy Lutomirski
2017-05-05 2:47 ` Jann Horn
2017-05-05 3:46 ` Linus Torvalds
2017-05-05 4:39 ` Al Viro
2017-05-05 4:44 ` Andy Lutomirski
2017-05-05 20:04 ` Eric W. Biederman
2017-05-05 20:28 ` Eric W. Biederman
2017-05-08 19:34 ` Mickaël Salaün
2017-05-18 8:50 ` David Drysdale
2017-09-10 20:26 Jürg Billeter
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