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([2001:a61:2518:4301:dfae:d37f:359:7ae8]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4854b0febd2sm17035925e9.32.2026.03.11.05.18.37 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 11 Mar 2026 05:18:38 -0700 (PDT) From: bauen1 X-Google-Original-From: bauen1 Message-ID: Date: Wed, 11 Mar 2026 13:18:36 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: Subject: x86/msr + lockdown: allow access to **documented** RAPL/TCC controls under Secure Boot To: "Rafael J. Wysocki" , "Artem S. Tashkinov" Cc: x86@kernel.org, Linux Kernel Mailing List , linux-pm@vger.kernel.org, linux-efi@vger.kernel.org, linux-security-module@vger.kernel.org, Srinivas Pandruvada , "Zhang, Rui" References: <2780abfc-39d1-4441-833c-65e66f747054@gmx.com> Content-Language: en-US Autocrypt: addr=j2468h@gmail.com; keydata= xsFNBFhYO0UBEADB9FOvBFPceReJkioc/Wpgb+4jquqgLaYFCq30wMRlbbxRE6W5piQdJBS9 1nHgehc1wKlpoX34I0fDYKmzhxU/wn7kPQqyIJ/x4Xc0un8rgLr6AB9J45+xYDAjTEP6wfzA DyCokyypi7knVSraYAUgmgBk+jEB/B1VpUxsE6X/tilqOLyPEkDX4dKUR/J2nPyfir3pYRFs siohNGbTOmwzwkA+rZClsUl9hO5n3oGAl3gJ352wIDJTDPd0YvyCTrHRpSTP9msKrFh3rILL aNgUNBr44QurGvxDuIrX6CIyqWUKO1tdnA1XOqsZDTEAa7IL6K7yoYRIzGZ+HmxemBhE/dxW qe4+nSru1QoucSNP6xa8F2HLeqvypD+xGerR4MELkBwa2XiGvS5OwF3XjevWcLQDztlXE1cW hK6fnK8XiXNcffG8YIhStSoW3dH3twPpEduqDAooLaCznxfNZFNcRU7iaoAk30xLv885jjga /FKs+jwlkzX/Xf6gvaLZhyIuF7x5yMFYZYKl/kA0XfY9x/d9YJe9MeBE5USZnssSGCgZXSt8 +tikDjEWAw43ANOG5Au/4wEoMI9eQmRRrQ9AfIb6MS1irfUwU0yGgHCkFX7nN54+2Zunvy9u YBk55oGh1MbVlIU/rEs+te0Syb8faX53oAMFPljqnqtS71AOLQARAQABzRliYXVlbjEgPGoy NDY4aEBnbWFpbC5jb20+wsGUBBMBCgA+AhsDBQsJCAcDBRUKCQgLBRYCAwEAAh4BAheAFiEE XtbYJqzUP47Z1Puy/wqvXggSupwFAmglfM4FCRGudQkACgkQ/wqvXggSupzqfxAApwDTP9Nc TMXdo6NYADg4T8P4AxMTCaVDL1iY2sHfV4sOr7tGCjrqbXMgdd2HVxl9joco1uYKKwmty7aR SOMeP9K+rrug/B0hhGwMn3enR7f1VdiWSwboD6tbfKnIOlRlzyPL2vLwtgzFmSI0fCvi2G3T /ve7okB3ABwlS3wz6fkUTmZecEKYgV5Nx9JZHZ7rSm/k5Xjp7vl5X4H15DJ1J2MSNmcU1nRF PyabthJRL/ejmXQossVagQEqTooF0xNIpEv2hmDMnU9+2dqc21PU3/D1MJ+jLNUP6HniymUn /nmStatTCnkWFQJtA5ayIdO89/KymP43vApU47ENIufubZEskMP5zjgemsdxW6od7TXE/43b ml5It1u9dqkGZbXnXvsWgIsMkM0hxAvW/So633Edhss1x+9lEiwKZpfALvZ6a4TsiO3Y+uro 5ZljHCL7reBj2QxQYO2kG15WHGATWpRnXazeA0VrcKqQp/Vs2InY/d2RgYBfWMp5cLS0QBqp s3c2nqI8RWr1Vd75UEzT5aWDMMk3XFkHXdPhllw1ySvejnRdcPxXLQ/RzFyBajoWT4QeAPYm UvyMbyIUewfsWHHqibtRDOROdpq8EyyLEwlajF/Udn05izIJna3xHsJ3ToxOLTsNWReHLkAa UyNm21lQ/s8AbWuMdlq4rh4M9DTOwU0EWFg7RQEQANuS3Qmbp63gCD7WHWWedBAY5t/FVrPR mf426pq2xAbms1WBHUeQB9r7F4fUMBFU03WNk8JWi4nSl8p0z4rZaZD1TEsenbYx2IohTxi0 qtZ/eaTydVzPfBIY3awBxaS3GuV8xUgR/8VdJATpEUF2BnDKGihXBl9pPM8l46vG6HsqWpeZ /hw/zwaGi8cSXY6PlFRL/fcpiGLR5RefH5VhDwZ5YrwDCYNhWYDKXL++IkDja0NW3s2yRUJM bRib0r8hq87lA7N+HHwgOOYd/sJbCZObZzL/n+lR+VTHLxGmJHbk+JRdagFH1l+x+Vp1zhVM XJDUci7Wcx/kCzCWu08t5t4Lef7rWvYJCf9JQaKJQcKyXr6ky3d4mYfV8AcA/9fat9NzQB6e 7cHw8yOc/1e4xN/h3cGNLWiGb8HCAR0SH22Gb2epyfq+txdn3cwm2ot2lhOXK3l48T081x/q kWOw86ig9dIVxi0RUv3CUaV0/N4SVumVD3GwzMSI0rfwuUb7tOqMGQFxe/k9Fc9uFPP7LfTe ZTOayuZg9oHO6Ju3x+KSXPwYcXAfuy0elZQPyqMZwshC3l1sfwG7Di+98sPzsbVUm9eTjTfN x2r7N/a958W0h+1SuE172qfuabLu8vMMWIuo8RaQG/OVF2bRR8yEPSyUTqS7Aj2osSX5CFB/ 4TVLABEBAAHCwXwEGAEKACYCGwwWIQRe1tgmrNQ/jtnU+7L/Cq9eCBK6nAUCaCV83AUJEa51 FwAKCRD/Cq9eCBK6nN4jD/9K6mi1SSVAa06QsMmTn5ix+PUkVu/ci/fEca/HByEBIg9X/Mvk zxlzAeUGcUNKK5//lPmc97H/iNZYE5KCbL5ikbXdSs2XWq4otW7gJyEUX2CpHcvv/9CMV2oZ dia9wa403slZ3shPkpaVx8txOxYAg3ErNCqQ94pTMB6XihY5zn1ePcSfHq+++hXV6nXXEj7E ZjPLyNxK2IwTZfEkHSCpdMUlcx/m6lihEVqMP5T1eIyNQuVS5hGI0K6exxPnGiHDXfty0SzP /WszKklCv1YkkOZFGDojURfHMbDK8sgSqyU7i5ZzYRFIOs+uP6F2nE+9aXKAtZg8CN6IDEBT 7ZN1dBkWpNFSkgfQm6YN+9gyB0OCQl50Q4qgd8l/i/XFVpPdsM0mF5Q8pBANLl/8mB8L+lLr FNbRvMK7yS5TbAI7xf8DQW9Ss0Hp9hXEoilmj9EDeM5UFUNhT72Qu5XJzazMuI52sye1tK6L KUDa1iGsZ0ziGbakhimgG5Bqq7jtnrjMmMBxOs5is+J/+XrU39AZiH669gxeUsPFbKKO/2qC lzFvrOxF2EdPDcFquFp9R4EMpOU9nAMz0g1a0w/Sq1kzvNgihu/1BDpSgJJECOcDprJUz1Ku zLiD1OBWWqgvYBJUXHLXvB5r5YQcn5ZqBcLCL0HtgHXv3OFtcZWVK2ZxXQ== In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 3/9/26 4:13 PM, Rafael J. Wysocki wrote: > On Mon, Mar 9, 2026 at 1:24 PM Artem S. Tashkinov wrote: >> >> Hello, >> >> When Secure Boot is enabled and kernel lockdown is active, the x86 MSR >> driver blocks all raw MSR access from user space via `/dev/cpu/*/msr`. >> This effectively prevents legitimate use of documented CPU power and >> thermal management interfaces such as RAPL power limits (PL1/PL2) and >> the TCC/TjOffset control. These registers are part of Intel’s >> **publicly** documented architectural interface and have been stable >> across many generations of processors. > > There is a power capping RAPL driver. What's the problem with it with > Secure Boot enabled? Hello, I believe that the comment about Secure Boot might come from the partially incorrect documentation of lockdown: https://lore.kernel.org/linux-security-module/20260203195001.20131-1-hi@alyssa.is/ > -On an EFI-enabled x86 or arm64 machine, lockdown will be automatically enabled > -if the system boots in EFI Secure Boot mode. > This is true for Fedora, where this page was sourced from, but I don't > believe it has ever been true for the mainline kernel, because Linus > rejected it. > >> As a result, under Secure Boot Linux users lose the ability to read or >> adjust **standard** power-management controls that remain available >> through equivalent tooling on other operating systems. > > The power capping RAPL driver is there, please use it. It is documented even. > > There is also a driver for TCC/TjOffset control, it is called intel_tcc_cooling. > > And there are utilities in user space (for example, Intel thermald) > that use those interfaces. > >> The current all-or-nothing restriction appears broader than necessary >> for the stated goal of protecting kernel integrity. MSRs associated with >> power limits and TCC offset are not privileged debugging or microcode >> interfaces but standard hardware configuration knobs intended for >> platform power and thermal management. >> >> It would be useful if the kernel either allowed access to a small >> whitelist of such documented registers under lockdown or exposed a >> mediated kernel interface for adjusting them. Without such a mechanism, >> Secure Boot effectively disables legitimate and widely used >> power/thermal tuning functionality on modern Intel laptops. >> >> Most (if not all) Intel laptops don't expose or allow to configure >> PL1/PL2 limits in BIOS/EFI either. > > Because it is not necessary to do so. > -- bauen1