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From:   Brian Gerst <brgerst@gmail.com>
Date:   Thu, 19 Dec 2019 22:48:13 -0500
Message-ID: <CAMzpN2if2m4McWpL49U4QAEM1MJ+qgTe-emN8vKcjVc1H+84vA@mail.gmail.com>
Subject: Re: [PATCH] x86: Remove force_iret()
To:     Andy Lutomirski <luto@kernel.org>
Cc:     X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Oleg Nesterov <oleg@redhat.com>
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On Thu, Dec 19, 2019 at 8:50 PM Andy Lutomirski <luto@kernel.org> wrote:
>
> On Thu, Dec 19, 2019 at 3:58 AM Brian Gerst <brgerst@gmail.com> wrote:
> >
> > force_iret() was originally intended to prevent the return to user mode with
> > the SYSRET or SYSEXIT instructions, in cases where the register state could
> > have been changed to be incompatible with those instructions.
>
> It's more than that.  Before the big syscall rework, we didn't restore
> the caller-saved regs.  See:
>
> commit 21d375b6b34ff511a507de27bf316b3dde6938d9
> Author: Andy Lutomirski <luto@kernel.org>
> Date:   Sun Jan 28 10:38:49 2018 -0800
>
>     x86/entry/64: Remove the SYSCALL64 fast path
>
> So if you changed r12, for example, the change would get lost.

force_iret() specifically dealt with changes to CS, SS and EFLAGS.
Saving and restoring the extra registers was a different problem
although it affected the same functions like ptrace, signals, and
exec.

--
Brian Gerst