Re: [PATCH v3 7/9] KVM: VMX: Add guest physical address check in EPT violation and misconfig

Sean Christopherson
2020-07-15T23:00:16Z

On Fri, Jul 10, 2020 at 05:48:09PM +0200, Mohammed Gamal wrote:
> Check guest physical address against it's maximum physical memory. If
> the guest's physical address exceeds the maximum (i.e. has reserved bits
> set), inject a guest page fault with PFERR_RSVD_MASK set.
>
> This has to be done both in the EPT violation and page fault paths, as
> there are complications in both cases with respect to the computation
> of the correct error code.
>
> For EPT violations, unfortunately the only possibility is to emulate,
> because the access type in the exit qualification might refer to an
> access to a paging structure, rather than to the access performed by
> the program.
>
> Trapping page faults instead is needed in order to correct the error code,
> but the access type can be obtained from the original error code and
> passed to gva_to_gpa. The corrections required in the error code are
> subtle. For example, imagine that a PTE for a supervisor page has a reserved
> bit set. On a supervisor-mode access, the EPT violation path would trigger.
> However, on a user-mode access, the processor will not notice the reserved
> bit and not include PFERR_RSVD_MASK in the error code.
>
> Co-developed-by: Mohammed Gamal <mgamal@redhat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx/vmx.c | 24 +++++++++++++++++++++---
> arch/x86/kvm/vmx/vmx.h | 3 ++-
> 2 files changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 770b090969fb..de3f436b2d32 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4790,9 +4790,15 @@ static int handle_exception_nmi(struct kvm_vcpu *vcpu)
>
> if (is_page_fault(intr_info)) {
> cr2 = vmx_get_exit_qual(vcpu);
> - /* EPT won't cause page fault directly */
> - WARN_ON_ONCE(!vcpu->arch.apf.host_apf_flags && enable_ept);
> - return kvm_handle_page_fault(vcpu, error_code, cr2, NULL, 0);
> + if (enable_ept && !vcpu->arch.apf.host_apf_flags) {
> + /*
> + * EPT will cause page fault only if we need to
> + * detect illegal GPAs.
> + */
> + kvm_fixup_and_inject_pf_error(vcpu, cr2, error_code);

This splats when running the PKU unit test, although the test still passed.
I haven't yet spent the brain power to determine if this is a benign warning,
i.e. simply unexpected, or if permission_fault() fault truly can't handle PK
faults.

WARNING: CPU: 25 PID: 5465 at arch/x86/kvm/mmu.h:197 paging64_walk_addr_generic+0x594/0x750 [kvm]
Hardware name: Intel Corporation WilsonCity/WilsonCity, BIOS WLYDCRB1.SYS.0014.D62.2001092233 01/09/2020
RIP: 0010:paging64_walk_addr_generic+0x594/0x750 [kvm]
Code: <0f> 0b e9 db fe ff ff 44 8b 43 04 4c 89 6c 24 30 8b 13 41 39 d0 89
RSP: 0018:ff53778fc623fb60 EFLAGS: 00010202
RAX: 0000000000000001 RBX: ff53778fc623fbf0 RCX: 0000000000000007
RDX: 0000000000000001 RSI: 0000000000000002 RDI: ff4501efba818000
RBP: 0000000000000020 R08: 0000000000000005 R09: 00000000004000e7
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000007
R13: ff4501efba818388 R14: 10000000004000e7 R15: 0000000000000000
FS: 00007f2dcf31a700(0000) GS:ff4501f1c8040000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000001dea475005 CR4: 0000000000763ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
PKRU: 55555554
Call Trace:
paging64_gva_to_gpa+0x3f/0xb0 [kvm]
kvm_fixup_and_inject_pf_error+0x48/0xa0 [kvm]
handle_exception_nmi+0x4fc/0x5b0 [kvm_intel]
kvm_arch_vcpu_ioctl_run+0x911/0x1c10 [kvm]
kvm_vcpu_ioctl+0x23e/0x5d0 [kvm]
ksys_ioctl+0x92/0xb0
__x64_sys_ioctl+0x16/0x20
do_syscall_64+0x3e/0xb0
entry_SYSCALL_64_after_hwframe+0x44/0xa9
---[ end trace d17eb998aee991da ]---