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From:   Kyle Huey <me@kylehuey.com>
Date:   Thu, 20 Aug 2020 10:26:47 -0700
Message-ID: <CAP045ApB9Ju+V2HsTb=z19Y5+AgB9eHhS_QEiv1E8cLdreewzw@mail.gmail.com>
Subject: Re: [REGRESSION] x86/entry: Tracer no longer has opportunity to
 change the syscall number at entry via orig_ax
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Kees Cook <keescook@chromium.org>,
        "Robert O'Callahan" <rocallahan@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        linux-arch@vger.kernel.org, Will Deacon <will@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Mark Rutland <mark.rutland@arm.com>,
        Keno Fischer <keno@juliacomputing.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        kvm list <kvm@vger.kernel.org>,
        Gabriel Krisman Bertazi <krisman@collabora.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>
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On Wed, Aug 19, 2020 at 12:44 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Wed, Aug 19 2020 at 10:14, Kyle Huey wrote:
> > tl;dr: after 27d6b4d14f5c3ab21c4aef87dd04055a2d7adf14 ptracer
> > modifications to orig_ax in a syscall entry trace stop are not honored
> > and this breaks our code.
>
> My fault and I have no idead why none of the silly test cases
> noticed. Fix below.

That'll do it, thanks.

- Kyle

> Thanks,
>
>         tglx
> ---
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 9852e0d62d95..fcae019158ca 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -65,7 +65,8 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
>
>         syscall_enter_audit(regs, syscall);
>
> -       return ret ? : syscall;
> +       /* The above might have changed the syscall number */
> +       return ret ? : syscall_get_nr(current, regs);
>  }
>
>  noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)