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    SubjectRe: [PATCH v2 05/11] KVM: x86: emulator: update the emulation mode after CR0 write
    On Tue, Jun 21, 2022, Maxim Levitsky wrote:
    > CR0.PE toggles real/protected mode, thus its update
    > should update the emulation mode.
    >
    > This is likely a benign bug because there is no writeback
    > of state, other than the RIP increment, and when toggling
    > CR0.PE, the CPU has to execute code from a very low memory address.
    >
    > Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
    > ---
    > arch/x86/kvm/emulate.c | 13 ++++++++++++-
    > 1 file changed, 12 insertions(+), 1 deletion(-)
    >
    > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
    > index 6f4632babc4cd8..002687d17f9364 100644
    > --- a/arch/x86/kvm/emulate.c
    > +++ b/arch/x86/kvm/emulate.c
    > @@ -3659,11 +3659,22 @@ static int em_movbe(struct x86_emulate_ctxt *ctxt)
    >
    > static int em_cr_write(struct x86_emulate_ctxt *ctxt)
    > {
    > - if (ctxt->ops->set_cr(ctxt, ctxt->modrm_reg, ctxt->src.val))
    > + int cr_num = ctxt->modrm_reg;
    > + int r;
    > +
    > + if (ctxt->ops->set_cr(ctxt, cr_num, ctxt->src.val))
    > return emulate_gp(ctxt, 0);
    >
    > /* Disable writeback. */
    > ctxt->dst.type = OP_NONE;
    > +
    > + if (cr_num == 0) {
    > + /* CR0 write might have updated CR0.PE */

    Or toggled CR0.PG. It's probably also worth noting that ->set_cr() handles side
    effects to other registers, e.g. the lack of an EFER.LMA update makes this look
    suspicious at first glance.

    > + r = update_emulation_mode(ctxt);
    > + if (r != X86EMUL_CONTINUE)
    > + return r;
    > + }
    > +
    > return X86EMUL_CONTINUE;
    > }
    >
    > --
    > 2.26.3
    >

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