On Mon, Oct 01, 2018 at 03:44:28PM +1000, Aleksa Sarai wrote:
On 2018-09-29, Jann Horn email@example.com wrote:
The problem is what happens if a folder you are walking through is concurrently moved out of the chroot. Consider the following scenario:
You attempt to open "C/../../etc/passwd" under the root "/A/B". Something else concurrently moves /A/B/C to /A/C. This can result in the following:
- You start the path walk and reach /A/B/C.
- The other process moves /A/B/C to /A/C. Your path walk is now at /A/C.
- Your path walk follows the first ".." up into /A. This is outside
the process root, but you never actually encountered the process root, so you don't notice. 4. Your path walk follows the second ".." up to /. Again, this is outside the process root, but you don't notice. 5. Your path walk walks down to /etc/passwd, and the open completes successfully. You now have an fd pointing outside your chroot.
If the root of your walk is below an attacker-controlled directory, this of course means that you lose instantly. If you point the root of the walk at a directory out of which a process in the container wouldn't be able to move the file, you're probably kinda mostly fine - as long as you know, for certain, that nothing else on the system would ever do that. But I still wouldn't feel good about that.
Please correct me if I'm wrong here (this is the first patch I've written for VFS). Isn't the retry/LOOKUP_REVAL code meant to handle this
Speaking naively, doesn't it make sense to invalidate the walk if a path component was modified? Or is this something that would be far too costly with little benefit?
Lookups and renames can definitely proceed in parallel, and yes I suspect it would be difficult to get good performance and guaranteed forward progress if you required lookup of the full path to be atomic with respect to renames.