From: Matthew Garrett Date: Wed, 8 Nov 2017 15:11:34 +0000 Subject: [13/29] x86/msr: Restrict MSR access when the kernel is locked down Origin: https://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-fs.git/commit?id=696dcddb285558b4febf318fe620a344d2b2fa47 Writing to MSRs should not be allowed if the kernel is locked down, since it could lead to execution of arbitrary code in kernel mode. Based on a patch by Kees Cook. MSR accesses are logged for the purposes of building up a whitelist as per Alan Cox's suggestion. Signed-off-by: Matthew Garrett Signed-off-by: David Howells Acked-by: Kees Cook Reviewed-by: Thomas Gleixner Reviewed-by: "Lee, Chun-Yi" cc: x86@kernel.org --- arch/x86/kernel/msr.c | 10 ++++++++++ 1 file changed, 10 insertions(+) Index: linux/arch/x86/kernel/msr.c =================================================================== --- linux.orig/arch/x86/kernel/msr.c +++ linux/arch/x86/kernel/msr.c @@ -84,6 +84,11 @@ static ssize_t msr_write(struct file *fi int err = 0; ssize_t bytes = 0; + if (kernel_is_locked_down("Direct MSR access")) { + pr_info("Direct access to MSR %x\n", reg); + return -EPERM; + } + if (count % 8) return -EINVAL; /* Invalid chunk size */ @@ -135,6 +140,11 @@ static long msr_ioctl(struct file *file, err = -EFAULT; break; } + if (kernel_is_locked_down("Direct MSR access")) { + pr_info("Direct access to MSR %x\n", regs[1]); /* Display %ecx */ + err = -EPERM; + break; + } err = wrmsr_safe_regs_on_cpu(cpu, regs); if (err) break;