Re: [PATCH] KVM: nVMX: properly pad struct kvm_vmx_nested_state_hdr

[Vitaly Kuznetsov]

Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes:

> On Mon, Jul 13, 2020 at 10:28:24AM +0200, Vitaly Kuznetsov wrote:
>> Holes in structs which are userspace ABI are undesireable.
>> 
>> Fixes: 83d31e5271ac ("KVM: nVMX: fixes for preemption timer migration")
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>>  Documentation/virt/kvm/api.rst  | 2 +-
>>  arch/x86/include/uapi/asm/kvm.h | 2 +-
>>  2 files changed, 2 insertions(+), 2 deletions(-)
>> 
>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>> index 320788f81a05..7beccda11978 100644
>> --- a/Documentation/virt/kvm/api.rst
>> +++ b/Documentation/virt/kvm/api.rst
>> @@ -4345,7 +4345,7 @@ Errors:
>>  	struct {
>>  		__u16 flags;
>>  	} smm;
>> -
>> +	__u16 pad;
>
> I don't think this is sufficient.  Before 83d31e5271ac, the struct was:
>

Before 850448f35aaf. Thanks, I was too lazy to check that.

> 	struct kvm_vmx_nested_state_hdr {
> 		__u64 vmxon_pa;
> 		__u64 vmcs12_pa;
>
> 		struct {
> 			__u16 flags;
> 		} smm;
> 	};
>
> which most/all compilers will pad out to 24 bytes on a 64-bit system.  And
> although smm.flags is padded to 8 bytes, it's initialized as a 2 byte value.
>
> 714             struct kvm_vmx_nested_state_hdr boo;
> 715             u64 val;
> 716
> 717             BUILD_BUG_ON(sizeof(boo) != 3*8);
> 718             boo.smm.flags = 0;
>    0xffffffff810148a9 <+41>:    xor    %eax,%eax
>    0xffffffff810148ab <+43>:    mov    %ax,0x18(%rsp)
>
> 719
> 720             val = *((volatile u64 *)(&boo.smm.flags));
>    0xffffffff810148b0 <+48>:    mov    0x18(%rsp),%rax
>
>
> Which means that userspace built for the old kernel will potentially send in
> garbage for the new 'flags' field due to it being uninitialized stack data,
> even with the layout after this patch.
>
> 	struct kvm_vmx_nested_state_hdr {
> 		__u64 vmxon_pa;
> 		__u64 vmcs12_pa;
>
> 		struct {
> 			__u16 flags;
> 		} smm;
> 		__u16 pad;
> 		__u32 flags;
> 		__u64 preemption_timer_deadline;
> 	};
>
> So to be backwards compatible I believe we need to add a __u32 pad as well,
> and to not cause internal padding issues, either make the new 'flags' a
> __u64 or pad that as well (or add and check a reserved __32).  Making flags
> a __64 seems like the least wasteful approach, e.g.
>
> 	struct kvm_vmx_nested_state_hdr {
> 		__u64 vmxon_pa;
> 		__u64 vmcs12_pa;
>
> 		struct {
> 			__u16 flags;
> 		} smm;
> 		__u16 pad16;
> 		__u32 pad32;
> 		__u64 flags;
> 		__u64 preemption_timer_deadline;
> 	};

I see and I agree but the fix like that needs to get into 5.8 or an ABI
breakage is guaranteed. I'll send v2 immediately, hope Paolo will take a
look.

>
>
>>  	__u32 flags;
>>  	__u64 preemption_timer_deadline;
>>    };
>> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
>> index 0780f97c1850..aae3df1cbd01 100644
>> --- a/arch/x86/include/uapi/asm/kvm.h
>> +++ b/arch/x86/include/uapi/asm/kvm.h
>> @@ -414,7 +414,7 @@ struct kvm_vmx_nested_state_hdr {
>>  	struct {
>>  		__u16 flags;
>>  	} smm;
>> -
>> +	__u16 pad;
>>  	__u32 flags;
>>  	__u64 preemption_timer_deadline;
>>  };
>> -- 
>> 2.25.4
>> 
>

-- 
Vitaly