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

[David Drysdale]

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.
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