Re: [PATCH] namei: permit linking with CAP_FOWNER in userns

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Dirk Steinmetz (public@rsjtdrjgfuzkfg.com):
> On Tue, 27 Oct 2015 09:33:44 -0500, Seth Forshee wrote:
> > 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>
> > 
> > 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.
> 
> I agree that this seems odd. I've chosen inode_owner_or_capable over
> capable_wrt_inode_uidgid(inode, CAP_FOWNER) as it seemed consistent:
> a privileged user (with CAP_SETUID) can impersonate the owner UID and thus
> bypass the check completely; this also matches the documented behavior of
> CAP_FOWNER: "Bypass permission checks on operations that normally require
> the filesystem UID of the process to match the UID of the file".
> 
> However, thinking about it I get the feeling that checking the gid seems
> reasonable as well. This is, however, independently of user namespaces.
> Consider the following scenario in any namespace, including the init one:
> - A file has the setgid and user/group executable bits set, and is owned
>   by user:group.
> - The user 'user' is not in the group 'group', and does not have any
>   capabilities.
> - The user 'user' hardlinks the file. The permission check will succeed,
>   as the user is the owner of the file.
> - The file is replaced with a newer version (for example fixing a security
>   issue)
> - Now user can still use the hardlink-pinned version to execute the file
>   as 'user:group' (and for example exploit the security issue).
> I would have expected the user to not be able to hardlink, as he lacks
> CAP_FSETID, and thus is not allowed to chmod, change or move the file
> without loosing the setgid bit. So it is impossible for him to make a non-
> hardlink copy with the setgid bit set -- why should he be able to make a
> hardlinked one?

Yeah, this sounds sensible.  It allows a user without access to 'disk',
for instance, to become that group.

> It seems to me as if may_linkat would additionally require a check
> verifying that either
> - not both setgid and group executable bit set
> - fsgid == owner gid
> - capable_wrt_inode_uidgid(CAP_FSETID) -- or CAP_FOWNER, depending on
>   whether to adapt chmod's behavior or keeping everything hardlink-
>   related in CAP_FOWNER; I don't feel qualified enough to pick ;)

In particular just changing it is not ok since people who are using file
capabilities to grant what they currently need would be stuck with a
mysterious new failure.

> This would change documented behavior (at least man proc.5's description
> of /proc/sys/fs/protected_hardlinks), and I'd consider it a separate
> issue, if any (as I'm unsure how realistic that scenario is). I'd
> appreciate comments on that.
> 
> For other situations than setgid-executable files I do not see issues with
> not checking the group id's mapping, as linking would be permitted without
> privileges outside of the user namespace (disregarding namespace-internal
> setuid bits).
> 
> Independently of that, it might be reasonable to consider switching
> inode_owner_or_capable towards checking the gid as well and define
> something along "uid checks in user namespaces with uid/gid maps require
> the file's uid and gid to be mapped, else they will fail" for consistency.
> 
> Dirk
>