On Tue, 17 Sep 2019 at 03:12, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Mon, Sep 16, 2019 at 03:12:49PM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Fixes: 0367f205a3b7 ("KVM: vmx: add support for emulating UMIP")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> > 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
