Re: [PATCH 1/3] fs: add O_BENEATH flag to openat(2)

[Andy Lutomirski]

On Wed, Nov 5, 2014 at 9:21 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
> On Tue, Nov 4, 2014 at 9:40 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
>> On Mon, Nov 3, 2014 at 5:22 PM, Eric W.Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>>> On November 3, 2014 7:42:58 AM PST, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>>On Mon, Nov 3, 2014 at 7:20 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>>>>wrote:
>>>>> On Mon, Nov 03, 2014 at 11:48:23AM +0000, David Drysdale wrote:
>>>>>> Add a new O_BENEATH flag for openat(2) which restricts the
>>>>>> provided path, rejecting (with -EACCES) paths that are not beneath
>>>>>> the provided dfd.  In particular, reject:
>>>>>>  - paths that contain .. components
>>>>>>  - paths that begin with /
>>>>>>  - symlinks that have paths as above.
>>>>>
>>>>> Yecch...  The degree of usefulness aside (and I'm not convinced that
>>>>it
>>>>> is non-zero),
>>>>
>>>>This is extremely useful in conjunction with seccomp.
>>>>
>>>>> WTF pass one bit out of nameidata->flags in a separate argument?
>>>>> Through the mutual recursion, no less...  And then you are not even
>>>>attempting
>>>>> to detect symlinks that are not followed by interpretation of _any_
>>>>pathname.
>>>>
>>>>How many symlinks like that are there?  Is there anything except
>>>>nd_jump_link users?  All of those are in /proc.  Arguably O_BENEATH
>>>>should prevent traversal of all of those links.
>>>
>>> Not commenting on the sanity of this one way or another, and I haven't read the patch.  There is an absolutely trivial implementation of this.
>>>
>>> After the path is resolved, walk backwards along d_parent and the mount tree, and see if you come to the file or directory dfd refers to.
>>>
>>> That can handle magic proc symlinks, and does not need to disallow .. or / explicitly so it should be much simpler code.
>>>
>>> My gut says that if Al says blech when looking at your code it is too complex to give you a security guarantee.
>>>
>>> Eric
>>
>> Well, the 'yecch' was deserved for the unnecessary duplication of the
>> flags.  Without that, the patch looks much simpler -- I'll send out a v2
>> with those changes for discussion, and think about your alternative
>> implementation suggestion (thanks!) separately.
>
> One concern with the "walk upwards and see if you get back where you
> started" approach -- it will allow use of a symlink that lives outside the
> original directory, but which points back inside it.  That's going to be
> slightly surprising behaviour for users, and I worry that there's the
> potential for unexpected information leakage from it.
>
> (BTW, size-wise my initial naive implementation of the walk-upward
> approach is only marginally smaller than the v2 patch.)

It has another problem.  Since we still haven't fixed the eternal
/proc/PID/fd-doesn't-respect-file-mode issue, you can have a read-only
fd somewhere and reopen it read-write using O_BENEATH on a different
fd.

For example:

fd 3 points to /sandbox
/sandbox/blocked has mode 0700 and isn't owned by us
/sandbox/blocked/foo has mode 0666
fd 4 points to /sandbox/blocked/foo, O_RDONLY

openat(3, "/proc/self/fd/4", O_RDWR | O_BENEATH) will get a read-write
descriptor pointing at /sandbox/blocked/foo, which should have been
impossible.

Also, I really don't like the information leak.  The result of asking
a server for "/home/victim/compromising-directory/../../www/index.html"
should not reveal whether compromising-directory exists.

--Andy
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