Date: Fri, 10 Nov 2017 12:43:47 +0900 From: Mahesh Bandewar (महेश बंडेवार) <maheshb@...gle.com> To: "Serge E. Hallyn" <serge@...lyn.com> Cc: Mahesh Bandewar <mahesh@...dewar.net>, LKML <linux-kernel@...r.kernel.org>, Netdev <netdev@...r.kernel.org>, Kernel-hardening <kernel-hardening@...ts.openwall.com>, Linux API <linux-api@...r.kernel.org>, Kees Cook <keescook@...omium.org>, "Eric W . Biederman" <ebiederm@...ssion.com>, Eric Dumazet <edumazet@...gle.com>, David Miller <davem@...emloft.net> Subject: Re: [PATCH resend 1/2] capability: introduce sysctl for controlled user-ns capability whitelist On Fri, Nov 10, 2017 at 2:30 AM, Serge E. Hallyn <serge@...lyn.com> wrote: > Quoting Mahesh Bandewar (mahesh@...dewar.net): >> From: Mahesh Bandewar <maheshb@...gle.com> >> >> Add a sysctl variable kernel.controlled_userns_caps_whitelist. This > > I understand the arguments in favor of whitelists in most cases for > security purposes. But given that you've said the goal here is to > prevent use of a capability in a user namespace when a CVE has been > found, a whitelist seems the wrong choice, since > > 1. it means that an attacker may through some other means be able > to add a capability back into the whitelist when you specifically > wanted to drop it. With a blacklist, you could say "once a cap has > been dropped it can never be re-added without rebooting". > 2. it means by default all capabilities will be denied once the > switch is pulled which is specifically not what you want in this > case. > 3. the admin can't just say "drop CAP_NET_ADMIN", but needs to > know to echo ~CAP_NET_ADMIN. > > Why not make it a blacklist, and once a cap is dropped it can > never be re-added? > Well, I'm not going to deny that blacklist approach would work equally well but code becomes little simpler when you use the whitelist approach. especially less complicated when a new capability needs to be added (not that we add capabilities very often) but that would be something one would have to pay attention to. However with this approach I can just the CAP_FULL_SET which is readily available. Having said that I specifically don't have strong preference in this regard (whitelist vs. blacklist). > -serge
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