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From:   Brian Gerst <brgerst@gmail.com>
Date:   Tue, 18 Jul 2023 10:25:59 -0400
Message-ID: <CAMzpN2jzsbUh=2JyCxvOCGJNpycaKPW9WL3JniwrzZB+-mf+tg@mail.gmail.com>
Subject: Re: [PATCH 2/6] x86/entry/64: Convert SYSRET validation tests to C
To:     =?UTF-8?Q?Mika_Penttil=C3=A4?= <mpenttil@redhat.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>
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On Tue, Jul 18, 2023 at 10:17=E2=80=AFAM Mika Penttil=C3=A4 <mpenttil@redha=
t.com> wrote:
>
> Hi,
>
>
> On 18.7.2023 16.44, Brian Gerst wrote:
> > Signed-off-by: Brian Gerst <brgerst@gmail.com>
> > ---
> >   arch/x86/entry/common.c        | 50 ++++++++++++++++++++++++++++++-
> >   arch/x86/entry/entry_64.S      | 55 ++-------------------------------=
-
> >   arch/x86/include/asm/syscall.h |  2 +-
> >   3 files changed, 52 insertions(+), 55 deletions(-)
> >
> > diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> > index 6c2826417b33..afe79c3f1c5b 100644
> > --- a/arch/x86/entry/common.c
> > +++ b/arch/x86/entry/common.c
> > @@ -70,8 +70,12 @@ 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)
> >   {
> > +     long rip;
> > +     unsigned int shift_rip;
> > +
> >       add_random_kstack_offset();
> >       nr =3D syscall_enter_from_user_mode(regs, nr);
> >
> > @@ -84,6 +88,50 @@ __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;
> > +
> > +     /* CS and SS must match the values set in MSR_STAR */
> > +     if (unlikely(regs->cs !=3D __USER_CS || regs->ss !=3D __USER_DS))
> > +             return false;
> > +
> > +     /*
> > +      * On Intel CPUs, SYSRET with non-canonical RCX/RIP will #GP
> > +      * in kernel space.  This essentially lets the user take over
> > +      * the kernel, since userspace controls RSP.
> > +      *
> > +      * Change top bits to match most significant bit (47th or 56th bi=
t
> > +      * depending on paging mode) in the address.
> > +      */
> > +     shift_rip =3D (64 - __VIRTUAL_MASK_SHIFT + 1);
>
> Should this be:
>
>         shift_rip =3D (64 - __VIRTUAL_MASK_SHIFT - 1);
> ?

I removed a set of parentheses, which switched the sign from -1 to +1.
I could put it back if that's less confusing.

Brian Gerst