On 2020-01-14, Al Viro viro@zeniv.linux.org.uk wrote:
On Sat, Jan 11, 2020 at 08:07:19AM +1100, Aleksa Sarai wrote:
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:
- Open the candidate directory.
- 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).
- Use the fd from (1) for all operations.
... except that foo/../bar *WILL* cross into the covering mount, on any kernel that supports ...at(2) at all, so I would be very cautious about any kind "hardening" claims in that case.
In the use-case I have, we would have full control over what the path being opened is (and thus you wouldn't open "foo/../bar"). But I agree that generally the MNT_EXPIRE solution is really non-ideal anyway.
Not to mention that we're still screwed when it comes to using magic-links (because if someone bind-mounts a magic-link over a magic-link there's absolutely no race-free way to be sure that we're traversing the right magic-link -- for that we'll need to have a different solution).
I'm not sure about Linus' proposal - it looks rather convoluted and we get a hard to describe twist of semantics in an area (procfs symlinks vs. mount traversal) on top of everything else in there...
Yeah, I agree.
- do you see any problems on your testcases with the current #fixes?
That's commit 7a955b7363b8 as branch tip.
I will take a quick look later today, but I'm currently at a conference.
- do you have any updates you would like to fold into stuff in
#work.openat2? Right now I have a local variant of #work.namei (with fairly cosmetical change compared to vfs.git one) that merges clean with #work.openat2; I would like to do any updates/fold-ins/etc. of #work.openat2 *before* doing a merge and continuing to work on top of the merge results...
Yes, there were two patches I sent a while ago[1]. I can re-send them if you like. The second patch switches open_how->mode to a u64, but I'm still on the fence about whether that makes sense to do...
[1]: https://lore.kernel.org/lkml/20191219105533.12508-1-cyphar@cyphar.com/