On 2020-01-14, Al Viro <viro@xxxxxxxxxxxxxxxxxx> 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: > > > > 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. > > ... 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. > 1) 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. > 2) 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@xxxxxxxxxx/ -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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