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From:   Brian Gerst <brgerst@gmail.com>
Date:   Sun, 23 Jul 2023 07:17:53 -0400
Message-ID: <CAMzpN2i0NjQ4A3wk_FMhEibr00Zwyk+1SndY3aZSa-paNGS-gg@mail.gmail.com>
Subject: Re: [PATCH v2 2/6] x86/entry/64: Convert SYSRET validation tests to C
To:     "Li, Xin3" <xin3.li@intel.com>
Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "x86@kernel.org" <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Lutomirski, Andy" <luto@kernel.org>,
        =?UTF-8?Q?Mika_Penttil=C3=A4?= <mpenttil@redhat.com>
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On Sun, Jul 23, 2023 at 5:53=E2=80=AFAM Li, Xin3 <xin3.li@intel.com> wrote:
>
>
> > @@ -84,6 +85,43 @@ __visible noinstr void do_syscall_64(struct pt_regs =
*regs, int
> > nr)
> >
> >       instrumentation_end();
> >       syscall_exit_to_user_mode(regs);
>
> Would it be better to make the following code a new function?
>
> And then the similar changes in patch 6 could be merged into the new
> function with #ifdef CONFIG_X86_64.
>
> > +
> > +     /*
> > +      * 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.
> > +      */
> > +     if (unlikely(!__is_canonical_address(regs->ip, __VIRTUAL_MASK_SHI=
FT +
> > 1)))
> > +             return false;
> > +
> > +     /*
> > +      * SYSRET cannot restore RF.  It can restore TF, but unlike IRET,
> > +      * restoring TF results in a trap from userspace immediately afte=
r
> > +      * SYSRET.
> > +      */
> > +     if (unlikely(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)))
> > +             return false;
> > +
> > +     /* Use SYSRET to exit to userspace */
> > +     return true;
> >  }
> >  #endif
> >

The tests are similar but not enough to combine them.  If
IA32_EMULATION is enabled, both versions are needed so one copy of the
function with #ifdefs won't work..

Brian Gerst