Re: [PATCH v3 2/4] hvm/svm: Enable Breakpoint events

From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Alexandru Stefan ISAILA <aisaila@bitdefender.com>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
Cc: "boris.ostrovsky@oracle.com" <boris.ostrovsky@oracle.com>,
	"tamas@tklengyel.com" <tamas@tklengyel.com>,
	"suravee.suthikulpanit@amd.com" <suravee.suthikulpanit@amd.com>,
	"jbeulich@suse.com" <jbeulich@suse.com>,
	"rcojocaru@bitdefender.com" <rcojocaru@bitdefender.com>
Subject: Re: [PATCH v3 2/4] hvm/svm: Enable Breakpoint events
Date: Wed, 14 Feb 2018 19:11:08 +0000	[thread overview]
Message-ID: <04caaad7-1c30-c5cd-fd4e-ed42cd18eedc@citrix.com> (raw)
In-Reply-To: <1f5b2521-1722-565a-d4cc-8dc67877f68c@citrix.com>

On 14/02/18 18:22, Andrew Cooper wrote:
> On 14/02/18 16:10, Alexandru Stefan ISAILA wrote:
>> On Lu, 2018-02-12 at 15:54 +0000, Andrew Cooper wrote:
>>> On 12/02/18 15:08, Alexandru Isaila wrote:
>>>> @@ -2619,14 +2634,31 @@ void svm_vmexit_handler(struct
>>>> cpu_user_regs *regs)
>>>>          break;
>>>>
>>>>      case VMEXIT_EXCEPTION_BP:
>>>> -        if ( !v->domain->debugger_attached )
>>>> -            goto unexpected_exit_type;
>>>> -        /* AMD Vol2, 15.11: INT3, INTO, BOUND intercepts do not
>>>> update RIP. */
>>>> -        if ( (inst_len = __get_instruction_length(v, INSTR_INT3))
>>>> == 0 )
>>>> +        inst_len = __get_instruction_length(v, INSTR_INT3);
>>> There are multiple ways of ending up with this vmexit, and INT3 is
>>> not
>>> the only way.
>>>
>>> The old code was somewhat broken (but only in the case that a
>>> debugger
>>> was attached), but now with  this introspection hook active,
>>> executing
>>> `0xcd 0x03` will end up crashing the domain because of a length
>>> mismatch
>>> looking for 0xcc.
>>>
>>> You need to inspect EXITINTINFO to work out what went on here, and
>>> distinguish INT3 from INT $3.
>>>
>>> Can I suggest that you run this unit test
>>> http://xenbits.xen.org/docs/xtf/test-swint-emulation.html under debug
>>> introspection an check that you get all expected events?  Every time
>>> we
>>> touch this code, we seem to break it :(
>>>
>>> ~Andrew
>>>
>> I've tested on Intel and AMD and I only get events on int3. Further
>> more, I don't think there is any way to use the vmcb->exitintinfo
>> because all the fields are 0 on the time of VMEXIT_EXCEPTION_BP. Did I
>> understand the test scenario correctly?
> Quite possibly, but now I'm even more confused.  I'll have a quick play.

Ok - after some investigation, executing `int $3` triggers VMEXIT_SWINT,
with the vector in EXITINFO1, as opposed to triggering VMEXIT_EXCP3,
except that we don't have INTERCEPT_SWINT active, so it completes
internally.

Therefore, in your patch, we do expect only ever to find an int3
triggering VMEXIT_EXCEPTION_BP.  Sorry for the noise.

However, do you mind rebasing the remainder of your series onto
staging?  It doesn't apply cleanly any more.

~Andrew

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