KVM: x86: Emulate split-lock access as a write in emulator

Emulate split-lock accesses as writes if split lock detection is on
to avoid #AC during emulation, which will result in a panic(). This
should never occur for a well-behaved guest, but a malicious guest can
manipulate the TLB to trigger emulation of a locked instruction[1].

More discussion can be found at [2][3].

[1] https://lkml.kernel.org/r/8c5b11c9-58df-38e7-a514-dc12d687b198@redhat.com
[2] https://lkml.kernel.org/r/20200131200134.GD18946@linux.intel.com
[3] https://lkml.kernel.org/r/20200227001117.GX9940@linux.intel.com

Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lkml.kernel.org/r/20200410115517.084300242@linutronix.de

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 027dfd278a973d4c62c41cd3fd5a2de2a0dc36c8..3bf2ecafd0270f9ab484ecb2b503d5afea2fd616 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5839,6 +5839,7 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
 {
        struct kvm_host_map map;
        struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
+       u64 page_line_mask;
        gpa_t gpa;
        char *kaddr;
        bool exchanged;
@@ -5853,7 +5854,16 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
            (gpa & PAGE_MASK) == APIC_DEFAULT_PHYS_BASE)
                goto emul_write;
 
-       if (((gpa + bytes - 1) & PAGE_MASK) != (gpa & PAGE_MASK))
+       /*
+        * Emulate the atomic as a straight write to avoid #AC if SLD is
+        * enabled in the host and the access splits a cache line.
+        */
+       if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
+               page_line_mask = ~(cache_line_size() - 1);
+       else
+               page_line_mask = PAGE_MASK;
+
+       if (((gpa + bytes - 1) & page_line_mask) != (gpa & page_line_mask))
                goto emul_write;
 
        if (kvm_vcpu_map(vcpu, gpa_to_gfn(gpa), &map))