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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Tue, 30 Jun 2020 09:48:22 -0700
Message-ID: <CALCETrWUHw2WSDaJrxrwVrXSGz-zR0N2R3cT06esKBCpqyoZaQ@mail.gmail.com>
Subject: Re: [PATCH v33 15/21] x86/vdso: Add support for exception fixup in
 vDSO functions
To:     Borislav Petkov <bp@alien8.de>
Cc:     Sean Christopherson <sean.j.christopherson@intel.com>,
        Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        X86 ML <x86@kernel.org>, linux-sgx@vger.kernel.org,
        LKML <linux-kernel@vger.kernel.org>,
        Jethro Beekman <jethro@fortanix.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        asapek@google.com, "Xing, Cedric" <cedric.xing@intel.com>,
        chenalexchen@google.com, conradparker@google.com,
        cyhanish@google.com, Dave Hansen <dave.hansen@intel.com>,
        "Huang, Haitao" <haitao.huang@intel.com>,
        Josh Triplett <josh@joshtriplett.org>,
        "Huang, Kai" <kai.huang@intel.com>,
        "Svahn, Kai" <kai.svahn@intel.com>, kmoy@google.com,
        Christian Ludloff <ludloff@google.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Neil Horman <nhorman@redhat.com>, npmccallum@redhat.com,
        puiterwijk@redhat.com, David Rientjes <rientjes@google.com>,
        Thomas Gleixner <tglx@linutronix.de>, yaozhangx@google.com
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On Tue, Jun 30, 2020 at 1:41 AM Borislav Petkov <bp@alien8.de> wrote:
>
> On Mon, Jun 29, 2020 at 11:00:55PM -0700, Sean Christopherson wrote:
> > E.g. the vDSO function should get the fixup even if userspace screws
> > up mmap() and invokes __vdso_sgx_enter_enclave() without being tagged
> > an SGX task.
>
> I sincerely hope you don't mean this seriously.
>
> Please add a member to task_struct which denotes that a task is an
> sgx task, test that member where needed and forget real quickly about
> running *any* *fixup* for unrelated tasks.

I don't see the problem.  If you call this magic vDSO function and get
a fault, it gets handled.  What's the failure mode?

>
> > No hard dependency, it's normal kernel code.  My reasoning for dropping it
> > in .../vdso was largely to co-locate it with vdso/extable.h due to the
> > dependency on the format of 'struct vdso_exception_table_entry'.
>
> A struct which you defined instead of simply using struct
> exception_table_entry even if it has a handler member which would remain
> unused?

Don't forget the cross-arch issue.  We need that structure to have
constant layout so that the -m32 build from the vDSO has the same
layout as in the kernel.

So my only actual objection to the patch is that there should probably
be a comment above the structure reminding people that it needs to
have the same layout on 32-bit, x32, and x86_64.  So maybe the entries
should be s32 instead of int, although this doesn't really matter.