linux-kernel - Re: [PATCH] KVM: X86: Fix warning in handle

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Date:   Tue, 17 Sep 2019 16:22:48 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "# v3 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH] KVM: X86: Fix warning in handle_desc

On Tue, 17 Sep 2019 at 03:12, Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Mon, Sep 16, 2019 at 03:12:49PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@...cent.com>
> >
> > Reported by syzkaller:
> >
> >       WARNING: CPU: 0 PID: 6544 at /home/kernel/data/kvm/arch/x86/kvm//vmx/vmx.c:4689 handle_desc+0x37/0x40 [kvm_intel]
> >       CPU: 0 PID: 6544 Comm: a.out Tainted: G           OE     5.3.0-rc4+ #4
> >       RIP: 0010:handle_desc+0x37/0x40 [kvm_intel]
> >       Call Trace:
> >        vmx_handle_exit+0xbe/0x6b0 [kvm_intel]
> >        vcpu_enter_guest+0x4dc/0x18d0 [kvm]
> >        kvm_arch_vcpu_ioctl_run+0x407/0x660 [kvm]
> >        kvm_vcpu_ioctl+0x3ad/0x690 [kvm]
> >        do_vfs_ioctl+0xa2/0x690
> >        ksys_ioctl+0x6d/0x80
> >        __x64_sys_ioctl+0x1a/0x20
> >        do_syscall_64+0x74/0x720
> >        entry_SYSCALL_64_after_hwframe+0x49/0xbe
> >
> > When CR4.UMIP is set, guest should have UMIP cpuid flag. Current
> > kvm set_sregs function doesn't have such check when userspace inputs
> > sregs values. SECONDARY_EXEC_DESC is enabled on writes to CR4.UMIP in
> > vmx_set_cr4 though guest doesn't have UMIP cpuid flag. The testcast
> > triggers handle_desc warning when executing ltr instruction since guest
> > architectural CR4 doesn't set UMIP. This patch fixes it by adding check
> > for guest UMIP cpuid flag when get sreg inputs from userspace.
> >
> > Reported-by: syzbot+0f1819555fbdce992df9@...kaller.appspotmail.com
> > Fixes: 0367f205a3b7 ("KVM: vmx: add support for emulating UMIP")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> > ---
> > Note: syzbot report link https://lkml.org/lkml/2019/9/11/799
> >
> >  arch/x86/kvm/x86.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index f7cfd8e..83288ba 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -8645,6 +8645,10 @@ static int kvm_valid_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
> >                       (sregs->cr4 & X86_CR4_OSXSAVE))
> >               return  -EINVAL;
> >
> > +     if (!guest_cpuid_has(vcpu, X86_FEATURE_UMIP) &&
> > +                     (sregs->cr4 & X86_CR4_UMIP))
>
> Assuming vmx_set_cr4() fails because nested_cr4_valid() fails, isn't this
> a generic problem with nested VMX that just happens to be visible because
> of the WARN_ON() in handle_desc()?
>
> In general, KVM lets userspace set broken combinations of CPUID vs. CRx so
> that it doesn't dictate ordering, e.g. __set_sregs() intentionally calls
> kvm_x86_ops->set_cr4() instead of kvm_set_cr4(), which has all the CPUID
> checks.
>
> The existing OSXSAVE check in kvm_valid_sregs() is more about ensuring
> host support (see commit 6d1068b3a985, "KVM: x86: invalid opcode oops on
> SET_SREGS with OSXSAVE bit set (CVE-2012-4461)").
>
> Given that both vmx_set_cr4() and svm_set_cr4() can return failure and
> cause __set_sregs() to silently fail, what about adding a new x86 ops to
> pre-check cr4, e.g. vm_x86_ops->is_valid_cr4(), and then WARN if set_cr4()
> fails during __set_sregs()?

I'm a little lazy, just extract CPUID and CR4 combination checking and
add it to __set_sregs() in new version.

    Wanpeng