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

From: "Jan Beulich" <JBeulich@suse.com>
To: aisaila@bitdefender.com
Cc: Tamas K Lengyel <tamas@tklengyel.com>,
	Wei Liu <wei.liu2@citrix.com>,
	Razvan Cojocaru <rcojocaru@bitdefender.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Paul Durrant <paul.durrant@citrix.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [PATCH v4 2/2] x86/emulate: Send vm_event from emulate
Date: Wed, 22 May 2019 03:56:13 -0600	[thread overview]
Message-ID: <5CE51CBD0200007800231438@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <20190520125454.14805-2-aisaila@bitdefender.com>

>>> 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"?

> 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?

> --- 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.

> @@ -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?

> +        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.

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"?

> @@ -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.

> @@ -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.

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.

Jan

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