Hi Pawan, kernel test robot noticed the following build warnings: [auto build test WARNING on 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1] url: https://github.com/intel-lab-lkp/linux/commits/Pawan-Gupta/x86-bugs-Add-asm-helpers-for-executing-VERW/20231024-161029 base: 05d3ef8bba77c1b5f98d941d8b2d4aeab8118ef1 patch link: https://lore.kernel.org/r/20231024-delay-verw-v2-4-f1881340c807%40linux.intel.com patch subject: [PATCH v2 4/6] x86/bugs: Use ALTERNATIVE() instead of mds_user_clear static key reproduce: (https://download.01.org/0day-ci/archive/20231026/202310260517.TrEGc1ZW-lkp@xxxxxxxxx/reproduce) If you fix the issue in a separate patch/commit (i.e. not just a new version of the same patch/commit), kindly add following tags | Reported-by: kernel test robot <lkp@xxxxxxxxx> | Closes: https://lore.kernel.org/oe-kbuild-all/202310260517.TrEGc1ZW-lkp@xxxxxxxxx/ All warnings (new ones prefixed by >>): >> Documentation/arch/x86/mds.rst:153: WARNING: Unexpected section title. vim +153 Documentation/arch/x86/mds.rst 141 142 When transitioning from kernel to user space the CPU buffers are flushed 143 on affected CPUs when the mitigation is not disabled on the kernel 144 command line. The mitigation is enabled through the feature flag 145 X86_FEATURE_CLEAR_CPU_BUF. 146 147 The mitigation is invoked just before transitioning to userspace after 148 user registers are restored. This is done to minimize the window in 149 which kernel data could be accessed after VERW e.g. via an NMI after 150 VERW. 151 152 Corner case not handled > 153 ^^^^^^^^^^^^^^^^^^^^^^^ 154 Interrupts returning to kernel don't clear CPUs buffers since the 155 exit-to-user path is expected to do that anyways. But, there could be 156 a case when an NMI is generated in kernel after the exit-to-user path 157 has cleared the buffers. This case is not handled and NMI returning to 158 kernel don't clear CPU buffers because: 159 160 1. It is rare to get an NMI after VERW, but before returning to userspace. 161 2. For an unprivileged user, there is no known way to make that NMI 162 less rare or target it. 163 3. It would take a large number of these precisely-timed NMIs to mount 164 an actual attack. There's presumably not enough bandwidth. 165 4. The NMI in question occurs after a VERW, i.e. when user state is 166 restored and most interesting data is already scrubbed. Whats left 167 is only the data that NMI touches, and that may or may not be of 168 any interest. 169 170 -- 0-DAY CI Kernel Test Service https://github.com/intel/lkp-tests/wiki
