From c3f08ed15047002677b8b1f9f5bc29ced3f87a6f Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Wed, 8 Jul 2020 21:51:59 +0200 Subject: [PATCH] x86/kvm/svm: Use uninstrumented wrmsrl() to restore GS On guest exit MSR_GS_BASE contains whatever the guest wrote to it and the first action after returning from the ASM code is to set it to the host kernel value. This uses wrmsrl() which is interesting at least. wrmsrl() is either using native_write_msr() or the paravirt variant. The XEN_PV code is uninteresting as nested SVM in a XEN_PV guest does not work. But native_write_msr() can be placed out of line by the compiler especially when paravirtualization is enabled in the kernel configuration. The function is marked notrace, but still can be probed if CONFIG_KPROBE_EVENTS_ON_NOTRACE is enabled. That would be a fatal problem as kprobe events use per-CPU variables which are GS based and would be accessed with the guest GS. Depending on the GS value this would either explode in colorful ways or lead to completely undebugable data corruption. Aside of that native_write_msr() contains a tracepoint which objtool complains about as it is invoked from the noinstr section. As this cannot run inside a XEN_PV guest there is no point in using wrmsrl(). Use native_wrmsrl() instead which is just a plain native WRMSR without tracing or anything else attached. Signed-off-by: Thomas Gleixner Acked-by: Juergen Gross Message-Id: <20200708195322.244847377@linutronix.de> Signed-off-by: Paolo Bonzini --- arch/x86/kvm/svm/svm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c index 71500e865c94..535ad311ad02 100644 --- a/arch/x86/kvm/svm/svm.c +++ b/arch/x86/kvm/svm/svm.c @@ -3369,7 +3369,7 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, __svm_vcpu_run(svm->vmcb_pa, (unsigned long *)&svm->vcpu.arch.regs); #ifdef CONFIG_X86_64 - wrmsrl(MSR_GS_BASE, svm->host.gs_base); + native_wrmsrl(MSR_GS_BASE, svm->host.gs_base); #else loadsegment(fs, svm->host.fs); #ifndef CONFIG_X86_32_LAZY_GS -- 2.11.0