On 2020-01-07, Linus Torvalds torvalds@linux-foundation.org wrote:
On Tue, Jan 7, 2020 at 7:13 PM Al Viro viro@zeniv.linux.org.uk wrote:
Another interesting question is whether we want O_PATH open to trigger automounts.
It does sound like they shouldn't, but as you say:
The thing is, we do *NOT* trigger them(or traverse mountpoints) at the starting point of lookups. I believe it's a mistake (and mine, at that), but I doubt that there's anything that can be done about it at that point. It's a user-visible behaviour [..]
Hmm. I wonder how set in stone that is. We may have two decades of history of not doing it at start point of lookups, but we do *not* have two decades of history of O_PATH.
So what I think we agree would be sane behavior would be for O_PATH opens to not trigger automounts (unless there's a slash at the end, whatever), but _do_ add the mount-point traversal to the beginning of lookups.
But only do it for the actual O_PATH fd case, not the cwd/root/non-O_PATH case.
That way we maintain original behavior: if somebody overmounts your cwd, you still see the pre-mount directory on lookups, because your cwd is "under" the mount.
But if you open a file with O_PATH, and somebody does a mount _afterwards_, the openat() will see that later mount and/or do the automount.
Don't you think that would be the more sane/obvious semantics of how O_PATH should work?
If I'm understanding this proposal correctly, this would be a problem for the libpathrs use-case -- if this is done then there's no way to avoid a TOCTOU with someone mounting and the userspace program checking whether something is a mountpoint (unless you have Linux >5.6 and RESOLVE_NO_XDEV). Today, you can (in theory) do it with MNT_EXPIRE:
1. Open the candidate directory. 2. umount2(MNT_EXPIRE) the fd. * -EINVAL means it wasn't a mountpoint when we got the fd, and the fd is a stable handle to the underlying directory. * -EAGAIN or -EBUSY means that it was a mountpoint or became a mountpoint after the fd was opened (we don't care about that, but fail-safe is better here). 3. Use the fd from (1) for all operations.
Don't get me wrong, I want to fix this issue *properly* by adding some new kernel features that allow us to avoid worrying about mounts-over-magiclinks -- but on old kernels (which libpathrs cares about) I would be worried about changes like this being backported resulting in it being not possible to implement the hardening I mentioned up-thread.