From: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>
To: Seth Forshee <seth.forshee@canonical.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
"Eric W. Biederman" <ebiederm@xmission.com>,
Andy Lutomirski <luto@amacapital.net>,
Kees Cook <keescook@chromium.org>,
Serge Hallyn <serge.hallyn@canonical.com>,
Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>
Subject: Re: [PATCH] namei: permit linking with CAP_FOWNER in userns
Date: Tue, 27 Oct 2015 19:08:31 +0100 [thread overview]
Message-ID: <20151027190831.70f71671@rsjtdrjgfuzkfg.com> (raw)
In-Reply-To: <20151027143344.GB132460@ubuntu-hedt>
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?
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 ;)
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
next prev parent reply other threads:[~2015-10-27 18:08 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-10-10 14:59 [PATCH] namei: permit linking with CAP_FOWNER in userns Dirk Steinmetz
2015-10-20 14:09 ` Dirk Steinmetz
2015-10-27 14:33 ` Seth Forshee
2015-10-27 18:08 ` Dirk Steinmetz [this message]
2015-10-27 20:28 ` Serge Hallyn
2015-10-28 15:07 ` Dirk Steinmetz
2015-10-28 17:33 ` Serge Hallyn
2015-11-02 15:10 ` Dirk Steinmetz
2015-11-02 18:02 ` Serge Hallyn
2015-11-02 19:57 ` Andy Lutomirski
2015-11-03 0:39 ` [RFC] namei: prevent sgid-hardlinks for unmapped gids Dirk Steinmetz
2015-11-03 15:44 ` Serge Hallyn
2015-11-03 18:20 ` Kees Cook
2015-11-03 23:21 ` Dirk Steinmetz
2015-11-03 23:29 ` Kees Cook
2015-11-04 6:58 ` Willy Tarreau
2015-11-04 17:59 ` Andy Lutomirski
2015-11-04 18:15 ` Willy Tarreau
2015-11-04 18:17 ` Andy Lutomirski
2015-11-04 18:28 ` Willy Tarreau
2015-11-06 21:59 ` Kees Cook
2015-11-06 22:30 ` Andy Lutomirski
2015-11-07 0:11 ` Kees Cook
2015-11-07 0:16 ` Kees Cook
2015-11-07 0:48 ` Andy Lutomirski
2015-11-07 5:05 ` Kees Cook
2015-11-08 2:02 ` Theodore Ts'o
2015-11-10 15:08 ` Jan Kara
2015-11-19 20:11 ` Kees Cook
2015-11-19 21:57 ` Andy Lutomirski
2015-11-19 22:02 ` Dave Chinner
2015-11-20 0:11 ` Kees Cook
2015-11-04 14:46 ` Serge E. Hallyn
2015-10-27 21:04 ` [PATCH] namei: permit linking with CAP_FOWNER in userns Eric W. Biederman
2015-11-03 17:51 ` Kees Cook
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