Date: Sun, 22 Jan 2017 00:41:05 +0100 From: Solar Designer <solar@...nwall.com> To: kernel-hardening@...ts.openwall.com Subject: Disable and lock Silicon Debug feature on modern Intel CPUs Hi, If anyone is looking for a tiny kernel hardening task, this may be one. OpenBSD just got this a week ago: https://freshbsd.org/commit/openbsd/f16aad7b540921691f7841ef8ccbb7e7ca22dfd1 "Disable and lock Silicon Debug feature on modern Intel CPUs This implements one of the countermeasures against using Direct Connect Interface (DCI) to debug CPUs via USB3 mentioned in the "Tapping into the core" talk at the 33c3: identify and disable the Silicon Debug feature found in Haswell and newer CPUs." (and I stole the first line of the commit message for this message's Subject - well, at least I do it with this attribution). Silicon Debug should probably be disabled and locked by default, but there should be a kernel parameter to avoid this. The rationale to do this is to provide a tiny bit of protection against some physical attacks on booted up and decrypted yet locked laptops, while also allowing the system owner's intentional use of Silicon Debug. As far as I can tell, so far Linux only got the CPUID feature bit: http://lists.openwall.net/linux-kernel/2015/07/19/228 and presumably reporting due to arch/x86/kernel/cpu/mkcapflags.sh. FreeBSD and NetBSD also seem to have just a check for the bit and its reporting, also since 2015. Alexander
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