linux-kernel - Re: [PATCH v2 2/4] capabilities: Add securebit to restrict userns caps

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Message-ID: <20240610130540.GC2193924@mail.hallyn.com>
Date: Mon, 10 Jun 2024 08:05:40 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Jonathan Calmels <jcalmels@...0.net>
Cc: "Serge E. Hallyn" <serge@...lyn.com>, brauner@...nel.org,
	ebiederm@...ssion.com, Jonathan Corbet <corbet@....net>,
	Paul Moore <paul@...l-moore.com>, James Morris <jmorris@...ei.org>,
	KP Singh <kpsingh@...nel.org>,
	Matt Bobrowski <mattbobrowski@...gle.com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	Stanislav Fomichev <sdf@...gle.com>, Hao Luo <haoluo@...gle.com>,
	Jiri Olsa <jolsa@...nel.org>, Luis Chamberlain <mcgrof@...nel.org>,
	Kees Cook <kees@...nel.org>, Joel Granados <j.granados@...sung.com>,
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	Jarkko Sakkinen <jarkko@...nel.org>,
	Stephen Smalley <stephen.smalley.work@...il.com>,
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	Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
	containers@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-doc@...r.kernel.org,
	linux-security-module@...r.kernel.org, bpf@...r.kernel.org,
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	selinux@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v2 2/4] capabilities: Add securebit to restrict userns
 caps

On Mon, Jun 10, 2024 at 02:46:06AM -0700, Jonathan Calmels wrote:
> On Sun, Jun 09, 2024 at 09:33:01PM GMT, Serge E. Hallyn wrote:
> > On Sun, Jun 09, 2024 at 03:43:35AM -0700, Jonathan Calmels wrote:
> > > This patch adds a new capability security bit designed to constrain a
> > > task’s userns capability set to its bounding set. The reason for this is
> > > twofold:
> > > 
> > > - This serves as a quick and easy way to lock down a set of capabilities
> > >   for a task, thus ensuring that any namespace it creates will never be
> > >   more privileged than itself is.
> > > - This helps userspace transition to more secure defaults by not requiring
> > >   specific logic for the userns capability set, or libcap support.
> > > 
> > > Example:
> > > 
> > >     # capsh --secbits=$((1 << 8)) --drop=cap_sys_rawio -- \
> > >             -c 'unshare -r grep Cap /proc/self/status'
> > >     CapInh: 0000000000000000
> > >     CapPrm: 000001fffffdffff
> > >     CapEff: 000001fffffdffff
> > >     CapBnd: 000001fffffdffff
> > >     CapAmb: 0000000000000000
> > >     CapUNs: 000001fffffdffff
> > 
> > But you are not (that I can see, in this or the previous patch)
> > keeping SECURE_USERNS_STRICT_CAPS in securebits on the next
> > level unshare.  Though I think it's ok, because by then both
> > cap_userns and cap_bset are reduced and cap_userns can't be
> > expanded.  (Sorry, just thinking aloud here)
> 
> Right this is safe to reset, but maybe we do keep it if the secbit is
> locked? This is kind of a special case compared to the other bits.

I don't think it would be worth the extra complication in the
secbits code, and it's semantically very different from the
cap_userns.

> > > +	/* Limit userns capabilities to our parent's bounding set. */
> > 
> > In the case of userns_install(), it will be the target user namespace
> > creator's bounding set, right?  Not "our parent's"?
> 
> Good point, I should reword this comment.