linux-kernel - Re: [PATCH v3 2/9] selftests/sgx: Produce static-pie executable for test enclave

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Message-ID: <b59e79dc-5157-8729-5e1c-d016f9f688ef@cs.kuleuven.be>
Date:   Wed, 23 Aug 2023 15:19:53 +0200
From:   Jo Van Bulck <jo.vanbulck@...kuleuven.be>
To:     "Huang, Kai" <kai.huang@...el.com>,
        "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH v3 2/9] selftests/sgx: Produce static-pie executable for
 test enclave

On 22.08.23 02:26, Huang, Kai wrote:

> ... I think only this build flag change should be done in  this patch, as
> described in the changelog.
> 
> Because ...

> ... if I am not missing anything, this chunk isn't needed for _this_ patch.  The
> old code can still produce the correct stack address. __encl_base is only needed
> for the next patch, thus the relevant change should be moved to the next patch.

I understand the confusion, but the reason I included this small change 
already in this commit is to make sure the commit compiles standalone. 
That is, when building the original assembly statement "lea 
(encl_stack)(%rbx), %rax" with -static-pie -fPIE, the linker complains 
about a relocation it cannot resolve:

/usr/local/bin/ld: /tmp/cchIWyfG.o: relocation R_X86_64_32S against 
`.data' can not be used when making a PIE object; recompile with -fPIE
collect2: error: ld returned 1 exit status

The problem is that only RIP-relative addressing is legit for local 
symbols, ie "encl_stack(%rip)". Hence, __encl_base is already needed 
here to calculate the stack address in the updated asm sequence in this 
patch.

Hope this helps clarifying!

> I honestly don't understand what's the purpose of this code change.  I believe
> this change can be done (because it looks there's no need push/pop %rbx in the
> first place), but again it should be in the next patch I suppose.

Thanks, the purpose indeed was merely to remove redundant code that is 
not needed. I see that it would be better to include this in a separate 
patch, so I'll update this in the next patch revision.

FWIW: if this is okay, while I'm on it, I'll also take a shot at 
removing remaining (unnecessary) assembly register cleansing code to 
make it more obvious that the test enclave is *not* exemplary secure, as 
per our earlier discussions. Ie in response to Dave's earlier comments 
that "The only patches I want for the kernel are to make the test 
enclave more *obviously* insecure." [1].

Best,
Jo

[1] 
https://lore.kernel.org/all/da0cfb1e-e347-f7f2-ac72-aec0ee0d867d@intel.com/