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. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
