linux-kernel - Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

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Date:   Thu, 23 Jul 2020 22:22:04 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>,
        Fenghua Yu <fenghua.yu@...el.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Weiny Ira <ira.weiny@...el.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        X86 ML <x86@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Vishal Verma <vishal.l.verma@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "open list\:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "open list\:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

Andy Lutomirski <luto@...capital.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’s .fault handler) changes
> 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’t 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’t 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