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[121.45.207.92]) by smtp.gmail.com with ESMTPSA id j67sm74431179pfc.72.2019.04.28.21.54.06 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Sun, 28 Apr 2019 21:54:07 -0700 (PDT) From: Daniel Axtens To: Matthew Garrett , Andrew Donnellan Cc: James Morris , LSM List , Linux Kernel Mailing List , David Howells , Linux API , Andy Lutomirski , linuxppc-dev , Michael Ellerman , cmr Subject: Re: [PATCH V32 01/27] Add the ability to lock down access to the running kernel image In-Reply-To: <87wojdy8ro.fsf@dja-thinkpad.axtens.net> References: <20190404003249.14356-1-matthewgarrett@google.com> <20190404003249.14356-2-matthewgarrett@google.com> <059c523e-926c-24ee-0935-198031712145@au1.ibm.com> <87wojdy8ro.fsf@dja-thinkpad.axtens.net> Date: Mon, 29 Apr 2019 14:54:03 +1000 Message-ID: <87tvehxvh0.fsf@dja-thinkpad.axtens.net> MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, >>> I'm thinking about whether we should lock down the powerpc xmon debug >>> monitor - intuitively, I think the answer is yes if for no other reason >>> than Least Astonishment, when lockdown is enabled you probably don't >>> expect xmon to keep letting you access kernel memory. >> >> The original patchset contained a sysrq hotkey to allow physically >> present users to disable lockdown, so I'm not super concerned about >> this case - I could definitely be convinced otherwise, though. So Mimi contacted me offlist and very helpfully provided me with a much better and less confused justification for disabling xmon in lockdown: On x86, physical presence (== console access) is a trigger to disable/enable lockdown mode. In lockdown mode, you're not supposed to be able to modify memory. xmon allows you to modify memory, and therefore shouldn't be allowed in lockdown. So, if you can disable lockdown on the console that's probably OK, but it should be specifically disabling lockdown, not randomly editing memory with xmon. Regards, Daniel