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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Andy Lutomirski <luto@amacapital.net>,
        Fenghua Yu <fenghua.yu@intel.com>
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Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions
In-Reply-To: <C03DA782-BD1A-42E3-B118-ABB34BC5F2AF@amacapital.net>
References: <20200723165204.GB77434@romley-ivt3.sc.intel.com> <C03DA782-BD1A-42E3-B118-ABB34BC5F2AF@amacapital.net>
Date:   Thu, 23 Jul 2020 22:22:04 +0200
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Andy Lutomirski <luto@amacapital.net> writes:

> Suppose some kernel code (a syscall or kernel thread) changes PKRS
> then takes a page fault. The page fault handler needs a fresh
> PKRS. Then the page fault handler (say a VMA=E2=80=99s .fault handler) ch=
anges
> PKRS.  The we get an interrupt. The interrupt *also* needs a fresh
> PKRS and the page fault value needs to be saved somewhere.
>
> So we have more than one saved value per thread, and thread_struct
> isn=E2=80=99t going to solve this problem.

A stack of 7 entries and an index needs 32bytes total which is a
reasonable amount and solves the problem including scheduling from #PF
nicely. Make it 15 and it's still only 64 bytes.

> But idtentry_state is also not great for a couple reasons.  Not all
> entries have idtentry_state, and the unwinder can=E2=80=99t find it for
> debugging. For that matter, the page fault logic probably wants to
> know the previous PKRS, so it should either be stashed somewhere
> findable or it should be explicitly passed around.
>
> My suggestion is to enlarge pt_regs.  The save and restore logic can
> probably be in C, but pt_regs is the logical place to put a register
> that is saved and restored across all entries.

Kinda, but that still sucks because schedule from #PF will get it wrong
unless you do extra nasties.

> Whoever does this work will have the delightful job of figuring out
> whether BPF thinks that the layout of pt_regs is ABI and, if so,
> fixing the resulting mess.
>
> The fact the new fields will go at the beginning of pt_regs will make
> this an entertaining prospect.

Good luck with all of that.

Thanks,

        tglx