2017-03-07 09:12-0800, Jim Mattson:
> I believe this behavior would be documented in the chipset data sheet
> rather than the SDM, since the chipset returns all 1s for an unclaimed
> read.
Good point, thanks. I'll add it as commit comment when applying.
I looked at some data sheets, but couldn't find where they say that :(
Emulating a (preferable userspace-configurable) fallback would need
deeper changes, though.
> Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>
>
> On Tue, Mar 7, 2017 at 8:51 AM, Radim Krčmář <rkrcmar@xxxxxxxxxx> wrote:
>> Before trying to do nested_get_page() in nested_vmx_merge_msr_bitmap(),
>> we have already checked that the MSR bitmap address is valid (4k aligned
>> and within physical limits). SDM doesn't specify what happens if the
>> there is no memory mapped at the valid address, but Intel CPUs treat the
>> situation as if the bitmap was configured to trap all MSRs.
>>
>> KVM already does that by returning false and a correct handling doesn't
>> need the guest-trigerrable warning that was reported by syzkaller:
>> (The warning was originally there to catch some possible bugs in nVMX.)
>>
>> ------------[ cut here ]------------
>> WARNING: CPU: 0 PID: 7832 at arch/x86/kvm/vmx.c:9709
>> nested_vmx_merge_msr_bitmap arch/x86/kvm/vmx.c:9709 [inline]
>> WARNING: CPU: 0 PID: 7832 at arch/x86/kvm/vmx.c:9709
>> nested_get_vmcs12_pages+0xfb6/0x15c0 arch/x86/kvm/vmx.c:9640
>> Kernel panic - not syncing: panic_on_warn set ...
>> CPU: 0 PID: 7832 Comm: syz-executor1 Not tainted 4.10.0+ #229
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>> __dump_stack lib/dump_stack.c:15 [inline]
>> dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>> panic+0x1fb/0x412 kernel/panic.c:179
>> __warn+0x1c4/0x1e0 kernel/panic.c:540
>> warn_slowpath_null+0x2c/0x40 kernel/panic.c:583
>> nested_vmx_merge_msr_bitmap arch/x86/kvm/vmx.c:9709 [inline]
>> nested_get_vmcs12_pages+0xfb6/0x15c0 arch/x86/kvm/vmx.c:9640
>> enter_vmx_non_root_mode arch/x86/kvm/vmx.c:10471 [inline]
>> nested_vmx_run+0x6186/0xaab0 arch/x86/kvm/vmx.c:10561
>> handle_vmlaunch+0x1a/0x20 arch/x86/kvm/vmx.c:7312
>> vmx_handle_exit+0xfc0/0x3f00 arch/x86/kvm/vmx.c:8526
>> vcpu_enter_guest arch/x86/kvm/x86.c:6982 [inline]
>> vcpu_run arch/x86/kvm/x86.c:7044 [inline]
>> kvm_arch_vcpu_ioctl_run+0x1418/0x4840 arch/x86/kvm/x86.c:7205
>> kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2570
>>
>> Reported-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
>> Signed-off-by: Radim Krčmář <rkrcmar@xxxxxxxxxx>
>> ---
>> arch/x86/kvm/vmx.c | 4 +---
>> 1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index ab338581b3ec..98e82ee1e699 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -9680,10 +9680,8 @@ static inline bool nested_vmx_merge_msr_bitmap(struct kvm_vcpu *vcpu,
>> return false;
>>
>> page = nested_get_page(vcpu, vmcs12->msr_bitmap);
>> - if (!page) {
>> - WARN_ON(1);
>> + if (!page)
>> return false;
>> - }
>> msr_bitmap_l1 = (unsigned long *)kmap(page);
>>
>> memset(msr_bitmap_l0, 0xff, PAGE_SIZE);
>> --
>> 2.12.0
>>
