Messages in this thread | | | From | (Eric W. Biederman) | Date | Tue, 27 Oct 2015 16:04:25 -0500 | Subject | Re: [PATCH] namei: permit linking with CAP_FOWNER in userns |
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Seth Forshee <seth.forshee@canonical.com> writes:
> On Tue, Oct 20, 2015 at 04:09:19PM +0200, Dirk Steinmetz wrote: >> Attempting to hardlink to an unsafe file (e.g. a setuid binary) from >> within an unprivileged user namespace fails, even if CAP_FOWNER is held >> within the namespace. This may cause various failures, such as a gentoo >> installation within a lxc container failing to build and install specific >> packages. >> >> This change permits hardlinking of files owned by mapped uids, if >> CAP_FOWNER is held for that namespace. Furthermore, it improves consistency >> by using the existing inode_owner_or_capable(), which is aware of >> namespaced capabilities as of 23adbe12ef7d3 ("fs,userns: Change >> inode_capable to capable_wrt_inode_uidgid"). >> >> Signed-off-by: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com> > > Tested-by: Seth Forshee <seth.forshee@canonical.com>
If multiple groups are hitting this issue for different reasons I am applying the supplied patch.
> This is hitting us in Ubuntu during some dpkg upgrades in containers. > When upgrading a file dpkg creates a hard link to the old file to back > it up before overwriting it. When packages upgrade suid files owned by a > non-root user the link isn't permitted, and the package upgrade fails. > This patch fixes our problem. > > I did want to point what seems to be an inconsistency in how > capabilities in user namespaces are handled with respect to inodes. When > I started looking at this my initial thought was to replace > capable(CAP_FOWNER) with capable_wrt_inode_uidgid(inode, CAP_FOWNER). On > the face of it this should be equivalent to what's done here, but it > turns out that capable_wrt_inode_uidgid requires that the inode's uid > and gid are both mapped into the namespace whereas > inode_owner_or_capable only requires the uid be mapped. I'm not sure how > significant that is, but it seems a bit odd.
It is a bit odd.
inode_owner_or_capable in this context is a gimme, as only being the owner of the file in question is enough to create a hard link, and root (in the user namespace) can become that user.
That said I think there have been some legitimate questions about setgid executables in may_linkat (raised down thread), as well as legitimate questions about capable_wrt_uidgid. I will add the additional question is it sane for us to ignore the acls in capable_wrt_uidgid.
All of this appears to be an area that no one except bad actors cares about so I expect we can change things without causing regressions, and on that note I encourage the conversation on the oddness to continue.
Eric
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