On Mon, Oct 23, 2023 at 07:58:57AM -0700, Sean Christopherson wrote: > On Fri, Oct 20, 2023, Pawan Gupta wrote: > > On Fri, Oct 20, 2023 at 03:55:07PM -0700, Sean Christopherson wrote: > > > On Fri, Oct 20, 2023, Pawan Gupta wrote: > > > > During VMentry VERW is executed to mitigate MDS. After VERW, any memory > > > > access like register push onto stack may put host data in MDS affected > > > > CPU buffers. A guest can then use MDS to sample host data. > > > > > > > > Although likelihood of secrets surviving in registers at current VERW > > > > callsite is less, but it can't be ruled out. Harden the MDS mitigation > > > > by moving the VERW mitigation late in VMentry path. > > > > > > > > Note that VERW for MMIO Stale Data mitigation is unchanged because of > > > > the complexity of per-guest conditional VERW which is not easy to handle > > > > that late in asm with no GPRs available. If the CPU is also affected by > > > > MDS, VERW is unconditionally executed late in asm regardless of guest > > > > having MMIO access. > > > > > > > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx> > > > > --- > > > > arch/x86/kvm/vmx/vmenter.S | 9 +++++++++ > > > > arch/x86/kvm/vmx/vmx.c | 10 +++++++--- > > > > 2 files changed, 16 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S > > > > index be275a0410a8..efa716cf4727 100644 > > > > --- a/arch/x86/kvm/vmx/vmenter.S > > > > +++ b/arch/x86/kvm/vmx/vmenter.S > > > > @@ -1,6 +1,7 @@ > > > > /* SPDX-License-Identifier: GPL-2.0 */ > > > > #include <linux/linkage.h> > > > > #include <asm/asm.h> > > > > +#include <asm/segment.h> > > > > #include <asm/bitsperlong.h> > > > > #include <asm/kvm_vcpu_regs.h> > > > > #include <asm/nospec-branch.h> > > > > @@ -31,6 +32,8 @@ > > > > #define VCPU_R15 __VCPU_REGS_R15 * WORD_SIZE > > > > #endif > > > > > > > > +#define GUEST_CLEAR_CPU_BUFFERS USER_CLEAR_CPU_BUFFERS > > > > + > > > > .macro VMX_DO_EVENT_IRQOFF call_insn call_target > > > > /* > > > > * Unconditionally create a stack frame, getting the correct RSP on the > > > > @@ -177,10 +180,16 @@ SYM_FUNC_START(__vmx_vcpu_run) > > > > * the 'vmx_vmexit' label below. > > > > */ > > > > .Lvmresume: > > > > + /* Mitigate CPU data sampling attacks .e.g. MDS */ > > > > + GUEST_CLEAR_CPU_BUFFERS > > > > > > I have a very hard time believing that it's worth duplicating the mitigation > > > for VMRESUME vs. VMLAUNCH just to land it after a Jcc. > > > > VERW modifies the flags, so it either needs to be after Jcc or we > > push/pop flags that adds 2 extra memory operations. Please let me know > > if there is a better option. > > Ugh, I assumed that piggybacking VERW overrode the original behavior entirely, I > didn't realize it sacrifices EFLAGS.ZF on the altar of mitigations. > > Luckily, this is easy to solve now that VMRESUME vs. VMLAUNCH uses a flag instead > of a dedicated bool. Thats great. > From: Sean Christopherson <seanjc@xxxxxxxxxx> > Date: Mon, 23 Oct 2023 07:44:35 -0700 > Subject: [PATCH] KVM: VMX: Use BT+JNC, i.e. EFLAGS.CF to select VMRESUME vs. > VMLAUNCH > > Use EFLAGS.CF instead of EFLAGS.ZF to track whether to use VMRESUME versus > VMLAUNCH. Freeing up EFLAGS.ZF will allow doing VERW, which clobbers ZF, > for MDS mitigations as late as possible without needing to duplicate VERW > for both paths. > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx> Thanks for the patch, I will include it in the next revision.
