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

Andy Lutomirski
2020-07-23T21:30:23Z

> On Jul 23, 2020, at 1:22 PM, Thomas Gleixner <tglx@linutronix.de> wrote:
>
> ﻿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’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.

This seems like we’re reinventing the wheel. PKRS is not
fundamentally different from, say, RSP. If we want to save it across
exceptions, we save it on entry and context-switch-out and restore it
on exit and context-switch-in.

>
>> 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.

We can always cheat like this:

struct real_pt_regs {
unsigned long pkrs;
struct pt_regs regs;
};

and pass a pointer to regs around. What BPF doesn't know about can't hurt it.