linux-kernel - Re: [PATCH 1/1] get_nr_restart_syscall() should return __NR_ia32_restart_syscall if __USER32

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Date:   Tue, 28 Mar 2017 10:10:30 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Jan Kratochvil <jan.kratochvil@...hat.com>,
        Pedro Alves <palves@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] get_nr_restart_syscall() should return
 __NR_ia32_restart_syscall if __USER32_CS

On Tue, Mar 28, 2017 at 9:27 AM, Oleg Nesterov <oleg@...hat.com> wrote:
> On 03/28, Andy Lutomirski wrote:
>>
>> On Tue, Mar 28, 2017 at 7:54 AM, Oleg Nesterov <oleg@...hat.com> wrote:
>> > get_nr_restart_syscall() checks TS_I386_REGS_POKED but this bit is only
>> > set if debugger is 32-bit. If a 64-bit debugger restores the registers
>> > of a 32-bit debugee outside of syscall exit path get_nr_restart_syscall()
>> > wrongly returns __NR_restart_syscall.
>>
>> I had sent a patch that introduced a new syscall nr, but it's not
>> quite safe because it could break seccomp-using programs.
>
> Ah, indeed...

This is, in theory, solvable.  It would be ugly and would pollute seccomp a bit.

>
>> But your
>> patch here is also screwy.
>
> Yes, yes, it doesn't try to solve all possible problems, I even mentioned
> this in the changelog.
>
>> How about we store the syscall arch to be restored in task_struct
>> along with restart_block?
>
> Yes, perhaps we will have to finally do this. Not really nice too.
>
>> the way there without heuristics as nasty as yours.
>
> I agree it will be better, but I refuse to treat them as mine checks ;)

:)

>
>> P.S. __USER32_CS is the wrong check even if we used your approach.
>> user_64bit_regs() is much better.
>
> Yes, thanks.  If only I understood what cs == pv_info.extra_user_64bit_cs
> actually means...
>

It means that, if Linux is a Xen PV guest, the GDT contains a bunch of
entries supplied by Xen and outside of Linux's control, and one of
those entries is a 64-bit DPL=3 code segment.  On the one hand, it's
annoying.  On the other hand, it serves a real purpose
performance-wise.

--Andy