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From:   Brian Gerst <brgerst@gmail.com>
Date:   Fri, 13 Oct 2023 09:05:56 -0400
Message-ID: <CAMzpN2gsRqO3eFtHFvCddNOWPQunuTc7W=OKxdGhqodq0oYQkg@mail.gmail.com>
Subject: Re: [PATCH v3 1/3] x86/entry/64: Convert SYSRET validation tests to C
To:     Nikolay Borisov <nik.borisov@suse.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>
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On Fri, Oct 13, 2023 at 8:39=E2=80=AFAM Nikolay Borisov <nik.borisov@suse.c=
om> wrote:
>
>
>
> On 12.10.23 =D0=B3. 1:43 =D1=87., Brian Gerst wrote:
> > Signed-off-by: Brian Gerst <brgerst@gmail.com>
> > ---
> >   arch/x86/entry/common.c        | 43 ++++++++++++++++++++++++++-
> >   arch/x86/entry/entry_64.S      | 53 ++-------------------------------=
-
> >   arch/x86/include/asm/syscall.h |  2 +-
> >   3 files changed, 45 insertions(+), 53 deletions(-)
> >
> > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > index 0551bcb197fb..207149a0a9b3 100644
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -71,7 +71,8 @@ static __always_inline bool do_syscall_x32(struct pt_=
regs *regs, int nr)
> >       return false;
> >   }
> >
> > -__visible noinstr void do_syscall_64(struct pt_regs *regs, int nr)
> > +/* Returns true to return using SYSRET, or false to use IRET */
> > +__visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
> >   {
> >       add_random_kstack_offset();
> >       nr =3D syscall_enter_from_user_mode(regs, nr);
> > @@ -85,6 +86,46 @@ __visible noinstr void do_syscall_64(struct pt_regs =
*regs, int nr)
> >
> >       instrumentation_end();
> >       syscall_exit_to_user_mode(regs);
> > +
> > +     /*
> > +      * Check that the register state is valid for using SYSRET to exi=
t
> > +      * to userspace.  Otherwise use the slower but fully capable IRET
> > +      * exit path.
> > +      */
> > +
> > +     /* XEN PV guests always use IRET path */
> > +     if (cpu_feature_enabled(X86_FEATURE_XENPV))
> > +             return false;
> > +
> > +     /* SYSRET requires RCX =3D=3D RIP and R11 =3D=3D EFLAGS */
> > +     if (unlikely(regs->cx !=3D regs->ip || regs->r11 !=3D regs->flags=
))
> > +             return false;
>
>
> Under what conditions do we expect this to not be true since we've come
> via the syscall which adheres to this layout? IOW isn't this always
> going to be true and we can simply eliminate it?

Any syscall that can modify pt_regs, like sigreturn(), exec(), etc.
Also ptrace can change registers.

Brian Gerst