linux-kernel - Re: RFC: userspace exception fixups

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Date:   Tue, 6 Nov 2018 09:00:38 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Andy Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Rich Felker <dalias@...c.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Jethro Beekman <jethro@...tanix.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Florian Weimer <fweimer@...hat.com>,
        Linux API <linux-api@...r.kernel.org>, X86 ML <x86@...nel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, nhorman@...hat.com,
        npmccallum@...hat.com, "Ayoun, Serge" <serge.ayoun@...el.com>,
        shay.katz-zamir@...el.com, linux-sgx@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Carlos O'Donell <carlos@...hat.com>,
        adhemerval.zanella@...aro.org
Subject: Re: RFC: userspace exception fixups

On 11/6/18 7:37 AM, Sean Christopherson wrote:
> 
> void *sgx_alloc_untrusted_stack(size_t size)
> {
> 	struct sgx_encl_tls *tls = get_encl_tls();
> 	struct sgx_out_call_context *context;
> 	void *tmp;
> 
> 	/* create a frame on the trusted stack to hold the out-call context */
> 	tls->trusted_stack -= sizeof(struct sgx_out_call_context);
> 
> 	/* save the untrusted %RSP into the out-call context */
> 	context = (struct sgx_out_call_context *)tls->trusted_stack;
> 	context->untrusted_stack = tls->save_state_area[SSA_RSP];
> 
> 	/* allocate space on the untrusted stack */
> 	tmp = (void *)(tls->save_state_area[SSA_RSP] - size);
> 	tls->save_state_area[SSA_RSP] = tmp;
> 
> 	return tmp;
> }

Why does it bother to go to all the trouble of mucking with the
untrusted stack?  It could *easily* just leave it alone and do out-calls
if it needs to allocate memory for parameter storage.  Heck, that could
theoretically even be _on_ the stack if the untrusted runtime was being
clever.

The only downside would be that the untrusted runtime would have to keep
track of the space a bit more explicitly so it could be cleaned up if
the enclave didn't do it.