Re: [PATCH v2 2/3] KVM: X86: Fix userspace set broken combinations of CPUID and CR4

[Wanpeng Li <kernellwp@gmail.com>]

On Wed, 18 Sep 2019 at 01:32, Sean Christopherson
<sean.j.christopherson@intel.com> wrote:
>
> On Tue, Sep 17, 2019 at 04:16:25PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@tencent.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 valid CR4 and CPUID combination checking in __set_sregs.
>
> Checking CPUID will fix this specific scenario, but it doesn't resolve
> the underlying issue of __set_sregs() ignoring the return of kvm_x86_ops'
> set_cr4(), e.g. I think vmx_set_cr4() can still fail if userspace sets a
> custom MSR_IA32_VMX_CR4_FIXED0 when nested VMX is on.
>
> > syzkaller source: https://syzkaller.appspot.com/x/repro.c?x=138efb99600000
> >
> > Reported-by: syzbot+0f1819555fbdce992df9@syzkaller.appspotmail.com
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
> > ---
> >  arch/x86/kvm/x86.c | 39 ++++++++++++++++++++++++---------------
> >  1 file changed, 24 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index f7cfd8e..cafb4d4 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -884,34 +884,42 @@ int kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr)
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_set_xcr);
> >
> > -int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> > +static int kvm_valid_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> >  {
> > -     unsigned long old_cr4 = kvm_read_cr4(vcpu);
> > -     unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE |
> > -                                X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE;
> > -
> > -     if (cr4 & CR4_RESERVED_BITS)
> > -             return 1;
> > -
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVE) && (cr4 & X86_CR4_OSXSAVE))
> > -             return 1;
> > +             return -EINVAL;
> >
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_SMEP) && (cr4 & X86_CR4_SMEP))
> > -             return 1;
> > +             return -EINVAL;
> >
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_SMAP) && (cr4 & X86_CR4_SMAP))
> > -             return 1;
> > +             return -EINVAL;
> >
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_FSGSBASE) && (cr4 & X86_CR4_FSGSBASE))
> > -             return 1;
> > +             return -EINVAL;
> >
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_PKU) && (cr4 & X86_CR4_PKE))
> > -             return 1;
> > +             return -EINVAL;
> >
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_LA57) && (cr4 & X86_CR4_LA57))
> > -             return 1;
> > +             return -EINVAL;
> >
> >       if (!guest_cpuid_has(vcpu, X86_FEATURE_UMIP) && (cr4 & X86_CR4_UMIP))
> > +             return -EINVAL;
> > +
> > +     return 0;
> > +}
> > +
> > +int kvm_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> > +{
> > +     unsigned long old_cr4 = kvm_read_cr4(vcpu);
> > +     unsigned long pdptr_bits = X86_CR4_PGE | X86_CR4_PSE | X86_CR4_PAE |
> > +                                X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_PKE;
> > +
> > +     if (cr4 & CR4_RESERVED_BITS)
> > +             return 1;
>
> Checking CPUID bits but allowing unconditionally reserved bits to be set
> feels wrong.
>
> Paolo, can you provide an "official" ruling on how KVM_SET_SREGS should
> interact with reserved bits?  It's not at all clear from the git history
> if skipping the checks was intentional or an oversight.
>
> The CR4_RESERVED_BITS check has been in kvm_set_cr4() since the beginning
> of time (commit 6aa8b732ca01, "[PATCH] kvm: userspace interface").
>
> The first CPUID check came later, in commit 2acf923e38fb ("KVM: VMX:
> Enable XSAVE/XRSTOR for guest"), but its changelog is decidedly unhelpful.
>
> > +
> > +     if (kvm_valid_cr4(vcpu, cr4))
> >               return 1;
> >
> >       if (is_long_mode(vcpu)) {
> > @@ -8675,7 +8683,8 @@ static int __set_sregs(struct kvm_vcpu *vcpu, struct kvm_sregs *sregs)
> >       struct desc_ptr dt;
> >       int ret = -EINVAL;
> >
> > -     if (kvm_valid_sregs(vcpu, sregs))
> > +     if (kvm_valid_sregs(vcpu, sregs) ||
>
> No need for a line break.  Even better, call kvm_valid_cr4() from
> kvm_valid_sregs(), e.g. the X86_FEATURE_XSAVE check in kvm_valid_sregs()
> is now redundant and can be dropped, and "return kvm_valid_cr4(...)" from
> kvm_valid_sregs() can likely be optimized into a tail call.

handle it in new version.

    Wanpeng