linux-kernel - Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX enclave call

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Date:   Tue, 6 Oct 2020 20:28:19 +0300
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Jethro Beekman <jethro@...tanix.com>, x86@...nel.org,
        linux-sgx@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Cedric Xing <cedric.xing@...el.com>, akpm@...ux-foundation.org,
        andriy.shevchenko@...ux.intel.com, asapek@...gle.com, bp@...en8.de,
        chenalexchen@...gle.com, conradparker@...gle.com,
        cyhanish@...gle.com, dave.hansen@...el.com, haitao.huang@...el.com,
        kai.huang@...el.com, kai.svahn@...el.com, kmoy@...gle.com,
        ludloff@...gle.com, luto@...nel.org, nhorman@...hat.com,
        npmccallum@...hat.com, puiterwijk@...hat.com, rientjes@...gle.com,
        tglx@...utronix.de, yaozhangx@...gle.com, mikko.ylinen@...el.com
Subject: Re: [PATCH v39 21/24] x86/vdso: Implement a vDSO for Intel SGX
 enclave call

On Tue, Oct 06, 2020 at 08:15:32AM -0700, Sean Christopherson wrote:
> On Tue, Oct 06, 2020 at 10:30:16AM +0200, Jethro Beekman wrote:
> > On 2020-10-06 04:57, Sean Christopherson wrote:
> > > On Sat, Oct 03, 2020 at 07:50:56AM +0300, Jarkko Sakkinen wrote:
> > >> +struct sgx_enclave_run {
> > >> +  __u64 tcs;
> > >> +  __u64 user_handler;
> > >> +  __u64 user_data;
> > >> +  __u32 leaf;
> > >
> > > I am still very strongly opposed to omitting exit_reason.  It is not at all
> > > difficult to imagine scenarios where 'leaf' alone is insufficient for the
> > > caller or its handler to deduce why the CPU exited the enclave.  E.g. see
> > > Jethro's request for intercepting interrupts.
> >
> > Not entirely sure what this has to do with my request, I just expect to see
> > leaf=ERESUME in this case, I think? E.g. as you would see in EAX when calling
> > ENCLU.
> 
> But how would you differentiate from the case that an exception occured in
> the enclave?  That will also transfer control with leaf=ERESUME.  If there
> was a prior exception and userspace didn't zero out the struct, there would
> be "valid" data in the exception fields.
> 
> An exit_reason also would allow retrofitting the exception fields into a
> union, i.e. the fields are valid if and only if exit_reason is exception.

Let's purge this a bit. Please remark where my logic goes wrong. I'm
just explaining how I've deduced the whole thing.

The information was encoded in v38 version of the vDSO was exactly this:

- On normal EEXIT, it got the value 0.
- Otherwise, it got the value 1.

The leaf, then embdded to struct sgx_exception but essentially the same
field got the value from EAX, and the value that EAX had was only
written on exception path.

Thus, I deduced that if you write $EEXIT to leaf on synchrous exit you
get the same information content, nothing gets overwritten. I.e. you
can make same conclusions as you would with those two struct fields.

/Jarkko