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

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Message-ID: <20170328162736.GA3983@redhat.com>
Date:   Tue, 28 Mar 2017 18:27:37 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 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...

> 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...


OK, please ignore this patch, I'll try to make another fix.

Oleg.