Virtualization - Feb 2021 - [PATCH 2/3] x86/sev-es: Check if regs->sp is trusted before adjusting #VC IST stack

Hi Andy,

On Wed, Feb 17, 2021 at 10:09:46AM -0800, Andy Lutomirski
wrote:
> Can you get rid of the linked list hack while you're at it?  This code
> is unnecessarily convoluted right now, and it seems to be just asking
> for weird bugs.  Just stash the old value in a local variable, please.
Yeah, the linked list is not really necessary right now, because of the
way nested NMI handling works and given that these functions are only
used in the NMI handler right now.
The whole #VC handling code was written with future requirements in
mind, like what is needed when debugging registers get virtualized and
#HV gets enabled.
Until its clear whether __sev_es_ist_enter/exit() is needed in any of
these paths, I'd like to keep the linked list for now. It is more
complicated but allows nesting.
> Meanwhile, I'm pretty sure I can break this whole scheme if the
> hypervisor is messing with us.  As a trivial example, the sequence
> SYSCALL gap -> #VC -> NMI -> #VC will go quite poorly.
I don't see how this would break, can you elaborate?

What I think happens is:

SYSCALL gap (RSP is from userspace and untrusted)

	-> #VC - Handler on #VC IST stack detects that it interrupted
	   the SYSCALL gap and switches to the task stack.


	-> NMI - Now running on NMI IST stack. Depending on whether the
	   stack switch in the #VC handler already happened, the #VC IST
	   entry is adjusted so that a subsequent #VC will not overwrite
	   the interrupted handlers stack frame.

	-> #VC - Handler runs on the adjusted #VC IST stack and switches
	   itself back to the NMI IST stack. This is safe wrt. nested
	   NMIs as long as nested NMIs itself are safe.

As a rule of thumb, think of the #VC handler as trying to be a non-IST
handler by switching itself to the interrupted stack or the task stack.
If it detects that this is not possible (which can't happen right now,
but with SNP), it will kill the guest.

Also #VC is currently not safe against #MC, but this is the same as with
NMI and #MC. And more care is needed when SNP gets enabled and #VCs can
happen in the stack switching/stack adjustment code itself. I will
probably just add a check then to kill the guest if an SNP related #VC
comes from noinstr code.
> Is this really better than just turning IST off for #VC and
> documenting that we are not secure against a malicious hypervisor yet?
It needs to be IST, even without SNP, because #DB is IST too. When the
hypervisor intercepts #DB then any #DB exception will be turned into
#VC, so #VC needs to be handled anywhere a #DB can happen.

And with SNP we need to be able to at least detect a malicious HV so we
can reliably kill the guest. Otherwise the HV could potentially take
control over the guest's execution flow and make it reveal its secrets.

Regards,

	Joerg

On Thu, Feb 18, 2021 at 3:25 AM Joerg Roedel <joro at 8bytes.org>
wrote:
>
> Hi Andy,
>
> On Wed, Feb 17, 2021 at 10:09:46AM -0800, Andy Lutomirski wrote:
> > Can you get rid of the linked list hack while you're at it?  This
code
> > is unnecessarily convoluted right now, and it seems to be just asking
> > for weird bugs.  Just stash the old value in a local variable, please.
>
> Yeah, the linked list is not really necessary right now, because of the
> way nested NMI handling works and given that these functions are only
> used in the NMI handler right now.
> The whole #VC handling code was written with future requirements in
> mind, like what is needed when debugging registers get virtualized and
> #HV gets enabled.
> Until its clear whether __sev_es_ist_enter/exit() is needed in any of
> these paths, I'd like to keep the linked list for now. It is more
> complicated but allows nesting.
I don't understand what this means.  The whole entry mechanism on x86
is structured so that we call a C function *and return from that C
function without longjmp-like magic* with the sole exception of
unwind_stack_do_exit().  This means that you can match up enters and
exits, and that unwind_stack_do_exit() needs to unwind correctly.  In
the former case, it's normal C and we can use normal local variables.
In the latter case, we know exactly what state we're trying to restore
and we can restore it directly without any linked lists or similar.

What do you have in mind that requires a linked list?
>
> > Meanwhile, I'm pretty sure I can break this whole scheme if the
> > hypervisor is messing with us.  As a trivial example, the sequence
> > SYSCALL gap -> #VC -> NMI -> #VC will go quite poorly.
>
> I don't see how this would break, can you elaborate?
>
> What I think happens is:
>
> SYSCALL gap (RSP is from userspace and untrusted)
>
>         -> #VC - Handler on #VC IST stack detects that it interrupted
>            the SYSCALL gap and switches to the task stack.
>
Can you point me to exactly what code you're referring to?  I spent a
while digging through the code and macro tangle and I can't find this.
>
>         -> NMI - Now running on NMI IST stack. Depending on whether the
>            stack switch in the #VC handler already happened, the #VC IST
>            entry is adjusted so that a subsequent #VC will not overwrite
>            the interrupted handlers stack frame.
>
>         -> #VC - Handler runs on the adjusted #VC IST stack and switches
>            itself back to the NMI IST stack. This is safe wrt. nested
>            NMIs as long as nested NMIs itself are safe.
>
> As a rule of thumb, think of the #VC handler as trying to be a non-IST
> handler by switching itself to the interrupted stack or the task stack.
> If it detects that this is not possible (which can't happen right now,
> but with SNP), it will kill the guest.
I will try to think of this, but it's hard, since I can't find the code
:)

I found this comment:

 * With the current implementation it is always possible to switch to a safe
 * stack because #VC exceptions only happen at known places, like intercepted
 * instructions or accesses to MMIO areas/IO ports. They can also happen with
 * code instrumentation when the hypervisor intercepts #DB, but the critical
 * paths are forbidden to be instrumented, so #DB exceptions currently also
 * only happen in safe places.

Unless AMD is more magic than I realize, the MOV SS bug^Wfeature means
that #DB is *not* always called in safe places.

But I *thnk* the code you're talking about is this:

    /*
     * If the entry is from userspace, switch stacks and treat it as
     * a normal entry.
     */
    testb    $3, CS-ORIG_RAX(%rsp)
    jnz    .Lfrom_usermode_switch_stack_\@

which does not run on #VC from kernel code.
> It needs to be IST, even without SNP, because #DB is IST too. When the
> hypervisor intercepts #DB then any #DB exception will be turned into
> #VC, so #VC needs to be handled anywhere a #DB can happen.
Eww.
>
> And with SNP we need to be able to at least detect a malicious HV so we
> can reliably kill the guest. Otherwise the HV could potentially take
> control over the guest's execution flow and make it reveal its secrets.
True.  But is the rest of the machinery to be secure against EFLAGS.IF
violations and such in place yet?
>
> Regards,
>
>         Joerg