Re: Testcases for "eip canonical" kvm fixes?

[Nadav Amit]

Automatic testings. I am working on a research paper regarding it, so Iâll
publish the details (hopefully) in the near future.

Nadav

Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> wrote:

> Hi,
> 
> I wonder how did you trigger the bug for your following fix?
> 
> 
> commit 234f3ce485d54017f15cf5e0699cff4100121601
> Author: Nadav Amit <namit@xxxxxxxxxxxxxxxxx>
> Date:   Thu Sep 18 22:39:38 2014 +0300
> 
>    KVM: x86: Emulator fixes for eip canonical checks on near branches
> 
>    Before changing rip (during jmp, call, ret, etc.) the target
> should be asserted
>    to be canonical one, as real CPUs do.  During sysret, both target
> rsp and rip
>    should be canonical. If any of these values is noncanonical, a #GP exception
>    should occur.  The exception to this rule are syscall and sysenter
> instructions
>    in which the assigned rip is checked during the assignment to the relevant
>    MSRs.
> 
>    This patch fixes the emulator to behave as real CPUs do for near branches.
>    Far branches are handled by the next patch.
> 
> 
> Non-canonical addresses result in #GP, but KVM doesn't
> intercept guest #GP, according to code below:
> 
> static void update_exception_bitmap(struct kvm_vcpu *vcpu)
> {
>        u32 eb;
> 
>        eb = (1u << PF_VECTOR) | (1u << UD_VECTOR) | (1u << MC_VECTOR) |
>             (1u << NM_VECTOR) | (1u << DB_VECTOR);
>        if ((vcpu->guest_debug &
>             (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)) ==
>            (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP))
>                eb |= 1u << BP_VECTOR;
>        if (to_vmx(vcpu)->rmode.vm86_active)
>                eb = ~0;
>        if (enable_ept)
>                eb &= ~(1u << PF_VECTOR); /* bypass_guest_pf = 0 */
>        if (vcpu->fpu_active)
>                eb &= ~(1u << NM_VECTOR);
> 
>        /* When we are running a nested L2 guest and L1 specified for it a
>         * certain exception bitmap, we must trap the same exceptions and pass
>         * them to L1. When running L2, we will only handle the exceptions
>         * specified above if L1 did not want them.
>         */
>        if (is_guest_mode(vcpu))
>                eb |= get_vmcs12(vcpu)->exception_bitmap;
> 
>        vmcs_write32(EXCEPTION_BITMAP, eb);
> }
> 
> 
> That what seems to be happening to me when I try to reproduce
> the bug on unpatched kernel: CPU simply emulates #GP, and
> everything works as it should.
> 
> In particular:
> I'm trying "RET far" with noncanonical address on stack (and valid CS).
> I expect that buggy behavior would be that #GP happens
> after "RET far". Instead, #GP happens _on_ "RET far" insn.
> 
> -- 
> vda


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