Re: [Xen-devel] [PATCH v4 2/2] x86/emulate: Send vm_event from emulate

[Alexandru Stefan ISAILA <aisaila@bitdefender.com>]

On 22.05.2019 12:56, Jan Beulich wrote:
>>>> On 20.05.19 at 14:55, <aisaila@bitdefender.com> wrote:
>> This patch aims to have mem access vm events sent from the emulator.
>> This is useful in the case of emulated instructions that cause
>> page-walks on access protected pages.
>>
>> We use hvmemul_map_linear_addr() ro intercept r/w access and
>> hvmemul_insn_fetch() to intercept exec access.
> 
> I'm afraid I don't understand this sentence. Or wait - is this a
> simple typo, and you mean "to" instead of "ro"?

Yes that is a typo it was meant to be a "to".

> 
>> First we try to send a vm event and if the event is sent then emulation
>> returns X86EMUL_ACCESS_EXCEPTION. If the event is not sent then the
>> emulation goes on as expected.
> 
> Perhaps it's obvious for a vm-event person why successful sending
> of an event is to result in X86EMUL_ACCESS_EXCEPTION, but it's not
> to me, despite having looked at prior versions. Can this (odd at the
> first glance) behavior please be briefly explained here?

If the event was successfully sent then the emulation has to stop and 
return.

> 
>> --- a/xen/arch/x86/hvm/emulate.c
>> +++ b/xen/arch/x86/hvm/emulate.c
>> @@ -15,6 +15,7 @@
>>   #include <xen/paging.h>
>>   #include <xen/trace.h>
>>   #include <xen/vm_event.h>
>> +#include <xen/monitor.h>
>>   #include <asm/event.h>
>>   #include <asm/i387.h>
>>   #include <asm/xstate.h>
>> @@ -26,6 +27,7 @@
>>   #include <asm/hvm/support.h>
>>   #include <asm/hvm/svm/svm.h>
>>   #include <asm/vm_event.h>
>> +#include <asm/altp2m.h>
> 
> In both cases please try to insert at least half way alphabetically
> (I didn't check if the directives are fully sorted already), rather
> than blindly at the end.

Ok, I will correct that.

> 
>> @@ -619,6 +621,68 @@ static int hvmemul_linear_to_phys(
>>       return X86EMUL_OKAY;
>>   }
>>   
>> +static bool hvmemul_send_vm_event(unsigned long gla,
>> +                                  uint32_t pfec, unsigned int bytes,
>> +                                  struct hvm_emulate_ctxt ctxt)
>> +{
>> +    xenmem_access_t access;
>> +    vm_event_request_t req = {};
>> +    gfn_t gfn;
>> +    paddr_t gpa;
>> +    unsigned long reps = 1;
>> +    int rc;
>> +
>> +    if ( !ctxt.send_event || !pfec )
> 
> Why the !pfec part of the condition?

Because it is used to check the type of access violation and if it is 0 
then we do not want to call get_mem_access or get the gpa, it is clear 
that there will be no violation.

> 
>> +        return false;
>> +
>> +    rc = hvmemul_linear_to_phys(gla, &gpa, bytes, &reps, pfec, &ctxt);
> 
> As said before - I don't think it's a good idea to do the page walk
> twice: This and the pre-existing one can easily return different
> results.

I do this just to get the gpa. If there is another way I will gladly use it.

> 
> Additionally, as also said before (I think), the function may raise
> #PF, which you don't seem to deal with despite discarding the
> X86EMUL_EXCEPTION return value ... >
>> +    if ( rc != X86EMUL_OKAY )
>> +        return false;
> 
> ... here.
> 
>> +    gfn = gaddr_to_gfn(gpa);
>> +
>> +    if ( p2m_get_mem_access(current->domain, gfn, &access,
>> +                            altp2m_vcpu_idx(current)) != 0 )
>> +        return false;
>> +
>> +    switch ( access ) {
>> +    case XENMEM_access_x:
>> +    case XENMEM_access_rx:
>> +        if ( pfec & PFEC_write_access )
>> +            req.u.mem_access.flags = MEM_ACCESS_R | MEM_ACCESS_W;
>> +        break;
>> +
>> +    case XENMEM_access_w:
>> +    case XENMEM_access_rw:
>> +        if ( pfec & PFEC_insn_fetch )
>> +            req.u.mem_access.flags = MEM_ACCESS_X;
>> +        break;
>> +
>> +    case XENMEM_access_r:
>> +    case XENMEM_access_n:
>> +        if ( pfec & PFEC_write_access )
>> +            req.u.mem_access.flags |= MEM_ACCESS_R | MEM_ACCESS_W;
>> +        if ( pfec & PFEC_insn_fetch )
>> +            req.u.mem_access.flags |= MEM_ACCESS_X;
>> +        break;
>> +
>> +    default:
>> +        return false;
>> +    }
> 
> Aren't you looking at the leaf page here, rather than at any of the
> involved page tables? Or am I misunderstanding the description
> saying "page-walks on access protected pages"?

We want to ignore access write for the page tables and only fire a 
vm_event for "regular" pages possibly hit by the actual instruction that 
has also happened to trigger the A/D write(s). So we don't want to send 
out vm_events for written-to page tables at all.

> 
>> @@ -636,6 +700,7 @@ static void *hvmemul_map_linear_addr(
>>       unsigned int nr_frames = ((linear + bytes - !!bytes) >> PAGE_SHIFT) -
>>           (linear >> PAGE_SHIFT) + 1;
>>       unsigned int i;
>> +    gfn_t gfn;
>>   
>>       /*
>>        * mfn points to the next free slot.  All used slots have a page reference
>> @@ -674,7 +739,7 @@ static void *hvmemul_map_linear_addr(
>>           ASSERT(mfn_x(*mfn) == 0);
>>   
>>           res = hvm_translate_get_page(curr, addr, true, pfec,
>> -                                     &pfinfo, &page, NULL, &p2mt);
>> +                                     &pfinfo, &page, &gfn, &p2mt);
>>   
>>           switch ( res )
>>           {
> 
> Are these two hunks leftovers? You don't use "gfn" anywhere.

Yes, there is no need for the gfn any more.

> 
>> @@ -1248,7 +1318,21 @@ int hvmemul_insn_fetch(
>>           container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
>>       /* Careful, as offset can wrap or truncate WRT insn_buf_eip. */
>>       uint8_t insn_off = offset - hvmemul_ctxt->insn_buf_eip;
>> +    uint32_t pfec = PFEC_page_present | PFEC_insn_fetch;
>> +    unsigned long addr, reps = 1;
>> +    int rc = 0;
>> +
>> +    rc = hvmemul_virtual_to_linear(
>> +        seg, offset, bytes, &reps, hvm_access_insn_fetch, hvmemul_ctxt, &addr);
>> +
>> +    if ( rc != X86EMUL_OKAY || !bytes )
>> +        return rc;
>> +
>> +    if ( hvmemul_ctxt->seg_reg[x86_seg_ss].dpl == 3 )
>> +        pfec |= PFEC_user_mode;
>>   
>> +    if ( hvmemul_send_vm_event(addr, pfec, bytes, *hvmemul_ctxt) )
>> +        return X86EMUL_ACCESS_EXCEPTION;
>>       /*
>>        * Fall back if requested bytes are not in the prefetch cache.
>>        * But always perform the (fake) read when bytes == 0.
> 
> Despite what was said before you're still doing things a 2nd time
> here just because of hvmemul_send_vm_event()'s needs, even
> if that function ends up bailing right away.

I don't understand what things are done 2 times. Can you please explain?

> 
> Also please don't lose the blank line ahead of the comment you
> add code ahead of.
> 
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.h
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.h
>> @@ -162,6 +162,8 @@ struct x86_emul_fpu_aux {
>>   #define X86EMUL_UNRECOGNIZED   X86EMUL_UNIMPLEMENTED
>>    /* (cmpxchg accessor): CMPXCHG failed. */
>>   #define X86EMUL_CMPXCHG_FAILED 7
>> +/* Emulator tried to access a protected page. */
>> +#define X86EMUL_ACCESS_EXCEPTION 8
> 
> This still doesn't make clear what the difference is to
> X86EMUL_EXCEPTION.

We need a return that has no side effects.

Alex
_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel