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

[David Drysdale]

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.)
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