On Tue, Nov 4, 2014 at 9:40 AM, David Drysdale <drysd...@google.com> wrote: > On Mon, Nov 3, 2014 at 5:22 PM, Eric W.Biederman <ebied...@xmission.com> > wrote: >> On November 3, 2014 7:42:58 AM PST, Andy Lutomirski <l...@amacapital.net> >> wrote: >>>On Mon, Nov 3, 2014 at 7:20 AM, Al Viro <v...@zeniv.linux.org.uk> >>>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.) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
