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Tue, 4 Feb 2020 17:14:50 +0000 Received: by smtp427.mail.ne1.yahoo.com (Oath Hermes SMTP Server) with ESMTPA ID 26caaa4df940cfe3c62d5a7f9873f224; Tue, 04 Feb 2020 17:14:46 +0000 (UTC) Subject: Re: [PATCH v14 22/23] LSM: Add /proc attr entry for full LSM context To: Stephen Smalley , casey.schaufler@intel.com, jmorris@namei.org, linux-security-module@vger.kernel.org, selinux@vger.kernel.org Cc: keescook@chromium.org, john.johansen@canonical.com, penguin-kernel@i-love.sakura.ne.jp, paul@paul-moore.com, Casey Schaufler References: <20200124002306.3552-1-casey@schaufler-ca.com> <20200124002306.3552-23-casey@schaufler-ca.com> <1de8338a-9c1c-c13b-16f0-e47ebec0e7ea@tycho.nsa.gov> <9afb8d9d-a590-0e13-bf46-53a347ea15dd@schaufler-ca.com> <6bd3e393-e1df-7117-d15a-81cb1946807b@tycho.nsa.gov> <446935fa-2926-c346-a273-ae1ecbb072cd@schaufler-ca.com> <09d96236-715a-344a-38bc-c05208698125@tycho.nsa.gov> <7ac0fa7d-225a-a554-bc1f-78d4d07b8adc@schaufler-ca.com> <9a07bf65-c0a8-6295-a3a5-ac1817385891@tycho.nsa.gov> From: Casey Schaufler Autocrypt: addr=casey@schaufler-ca.com; 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WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.4.2 MIME-Version: 1.0 In-Reply-To: <9a07bf65-c0a8-6295-a3a5-ac1817385891@tycho.nsa.gov> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Content-Language: en-US X-Mailer: WebService/1.1.15158 hermes Apache-HttpAsyncClient/4.1.4 (Java/1.8.0_181) Sender: owner-linux-security-module@vger.kernel.org Precedence: bulk List-ID: On 2/4/2020 5:37 AM, Stephen Smalley wrote: > On 2/3/20 4:39 PM, Casey Schaufler wrote: >> On 2/3/2020 11:37 AM, Stephen Smalley wrote: >>> BTW, I think the above guarantees with the exception of no trailing w= hitespace and whether the NUL byte is included or excluded from length ar= e in reality also required for "current" (and SO_PEERSEC) or existing use= rspace will break. >> >> The behavior of interfaces (e.g. "current", "exec") that are module de= fined >> is only of concern with respect to to "display" behavior. If a securit= y module >> wants to provide a variable size binary blob in "current" I would obje= ct on >> principle, but policy as I understand it has long been that if the aut= hors want >> to do that, it's their call. > > Doing so would break existing userspace (not just LSM-specific userspac= e), so I think it would be a deal breaker even for new security modules t= o move away from those guarantees for "current" at least. procps-ng (and = I think procps before it) directly reads /proc/pid/attr/current and trunc= ates at the first unprintable character.=20 An user-space that makes invalid assumptions about an interface can't implicitly define the behavior of that interface. You can't declare that "current" is defined to be a string just because a developer looked at how one security module uses it and coded the application accordingly. You can't declare that "current" will always be a SELinux context. That horse left the barn in 2007, and there are still people writing code assuming that is what they're getting. If the sub-system maintainer, James Morris, is willing to state that "current" must have a particular format that would be different. > systemd's sd-bus reads /proc/pid/attr/current directly and treats \n, \= r, or \0 byte as terminators and truncated on first occurrence.=C2=A0 A v= ariety of userspace code uses the value obtained from /proc/pid/attr/curr= ent and/or SO_PEERSEC as something that it can pass to printf-like functi= ons using a %s specifier for inclusion in logs and audit messages. Yup. And so far no security module has been foolish enough to export a binary blob in a /proc/.../attr interface. That doesn't mean that the interface is defined as a string. It's certainly the convention, but nowhere is it documented as a requirement. That's why I'm putting in the effort to define the format for "context" and SO_PEERCONTEXT. Interfaces at the LSM level need to be defined so as to allow the underlying security modules to provide the information they want in a way that is unambiguous and application non-hostile. The help I've gotten from you and the rest of the reviewers over the life of this effort has been extremely helpful, if not always easy to swallow. > >> The "context" has a defined format, and it would >> be up to the authors to come up with a printable ASCII representation = of the >> binary blob. If they care. They're not required to provide a "context"= =2E > >