linux-kernel - Re: [PATCH v26 10/22] x86/sgx: Linux Enclave Driver

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Message-ID: <2c077197-a8a7-feac-58ea-e901c92fb58b@linux.microsoft.com>
Date:   Thu, 20 Feb 2020 16:11:04 -0800
From:   Jordan Hand <jorhand@...ux.microsoft.com>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-sgx@...r.kernel.org, akpm@...ux-foundation.org,
        dave.hansen@...el.com, nhorman@...hat.com, npmccallum@...hat.com,
        haitao.huang@...el.com, andriy.shevchenko@...ux.intel.com,
        tglx@...utronix.de, kai.svahn@...el.com, bp@...en8.de,
        josh@...htriplett.org, luto@...nel.org, kai.huang@...el.com,
        rientjes@...gle.com, cedric.xing@...el.com, puiterwijk@...hat.com,
        linux-security-module@...r.kernel.org,
        Suresh Siddha <suresh.b.siddha@...el.com>,
        Haitao Huang <haitao.huang@...ux.intel.com>
Subject: Re: [PATCH v26 10/22] x86/sgx: Linux Enclave Driver

On 2/20/20 2:16 PM, Jarkko Sakkinen wrote:
> On Thu, Feb 20, 2020 at 10:48:42AM -0800, Sean Christopherson wrote:
>> My biggest concern for allowing PROT_EXEC if RIE is that it would result
>> in #PF(SGX) (#GP on Skylake) due to an EPCM violation if the enclave
>> actually tried to execute from such a page.  This isn't a problem for the
>> kernel as the fault will be reported cleanly through the vDSO (or get
>> delivered as a SIGSEGV if the enclave isn't entered through the vDSO), but
>> it's a bit weird for userspace as userspace will see the #PF(SGX) and
>> likely assume the EPC was lost, e.g. silently restart the enclave instead
>> of logging an error that the enclave is broken.
> 
> I think right way to fix the current implementation is to -EACCES mmap()
> (and mprotect) when !!(current->personality & READ_IMPLIES_EXEC).
> 

I agree. It still means userspace code with an executable stack can't
mmap/mprotect enclave pages and request PROT_READ but the check you've
proposed would more consistently enforce this which is easier to
understand from userspace perspective.

-Jordan