linux-kernel - Re: [PATCH v3 03/25] x86/sgx: Wipe out EREMOVE from sgx_free_epc

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Date:   Tue, 23 Mar 2021 16:21:47 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Kai Huang <kai.huang@...el.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-sgx@...r.kernel.org,
        linux-kernel@...r.kernel.org, jarkko@...nel.org, luto@...nel.org,
        dave.hansen@...el.com, rick.p.edgecombe@...el.com,
        haitao.huang@...el.com, pbonzini@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, hpa@...or.com
Subject: Re: [PATCH v3 03/25] x86/sgx: Wipe out EREMOVE from
 sgx_free_epc_page()

On Tue, Mar 23, 2021, Borislav Petkov wrote:
> On Tue, Mar 23, 2021 at 03:45:14PM +0000, Sean Christopherson wrote:
> > Practically speaking, "basic" deployments of SGX VMs will be insulated from
> > this bug.  KVM doesn't support EPC oversubscription, so even if all EPC is
> > exhausted, new VMs will fail to launch, but existing VMs will continue to chug
> > along with no ill effects....
> 
> Ok, so it sounds to me like *at* *least* there should be some writeup in
> Documentation/ explaining to the user what to do when she sees such an
> EREMOVE failure, perhaps the gist of this thread and then possibly the
> error message should point to that doc.
> 
> We will of course have to revisit when this hits the wild and people
> start (or not) hitting this. But judging by past experience, if it is
> there, we will hit it. Murphy says so.

I like the idea of pointing at the documentation.  The documentation should
probably emphasize that something is very, very wrong.  E.g. if a kernel bug
triggers EREMOVE failure and isn't detected until the kernel is widely deployed
in a fleet, then the folks deploying the kernel probably _should_ be in all out
panic.  For this variety of bug to escape that far, it means there are huge
holes in test coverage, in both the kernel itself and in the infrasturcture of
whoever is rolling out their new kernel.