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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 8 Nov 2018 13:22:54 -0800
Message-ID: <CALCETrUKzXYzRrWRdi8Z7AdAF0uZW5Gs7J4s=55dszoyzc29rw@mail.gmail.com>
Subject: Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for
 shadow stack
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To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>, "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, Balbir Singh <bsingharora@gmail.com>, Cyrill Gorcunov <gorcunov@gmail.com>, Dave Hansen <dave.hansen@linux.intel.com>, Eugene Syromiatnikov <esyr@redhat.com>, Florian Weimer <fweimer@redhat.com>, "H. J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>, Kees Cook <keescook@chromium.org>, Mike Kravetz <mike.kravetz@oracle.com>, Nadav Amit <nadav.amit@gmail.com>, Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>, Peter Zijlstra <peterz@infradead.org>, Randy Dunlap <rdunlap@infradead.org>, "Ravi V. Shankar" <ravi.v.shankar@intel.com>, "Shanbhogue, Vedvyas" <vedvyas.shanbhogue@intel.com>

On Thu, Nov 8, 2018 at 1:06 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> On Thu, 2018-11-08 at 12:46 -0800, Andy Lutomirski wrote:
> > On Thu, Oct 11, 2018 at 8:20 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> > >
> > > Intel Control-flow Enforcement Technology (CET) introduces the
> > > following MSRs into the XSAVES system states.
> > >
> > >     IA32_U_CET (user-mode CET settings),
> > >     IA32_PL3_SSP (user-mode shadow stack),
> > >     IA32_PL0_SSP (kernel-mode shadow stack),
> > >     IA32_PL1_SSP (ring-1 shadow stack),
> > >     IA32_PL2_SSP (ring-2 shadow stack).
> > >
> > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > > ---
> > >  arch/x86/include/asm/fpu/types.h            | 22 +++++++++++++++++++++
> > >  arch/x86/include/asm/fpu/xstate.h           |  4 +++-
> > >  arch/x86/include/uapi/asm/processor-flags.h |  2 ++
> > >  arch/x86/kernel/fpu/xstate.c                | 10 ++++++++++
> > >  4 files changed, 37 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/include/asm/fpu/types.h
> > > b/arch/x86/include/asm/fpu/types.h
> > > index 202c53918ecf..e55d51d172f1 100644
> > > --- a/arch/x86/include/asm/fpu/types.h
> > > +++ b/arch/x86/include/asm/fpu/types.h
> > > @@ -114,6 +114,9 @@ enum xfeature {
> > >         XFEATURE_Hi16_ZMM,
> > >         XFEATURE_PT_UNIMPLEMENTED_SO_FAR,
> > >         XFEATURE_PKRU,
> > > +       XFEATURE_RESERVED,
> > > +       XFEATURE_SHSTK_USER,
> > > +       XFEATURE_SHSTK_KERNEL,
> > >
> > >         XFEATURE_MAX,
> > >  };
> > > @@ -128,6 +131,8 @@ enum xfeature {
> > >  #define XFEATURE_MASK_Hi16_ZMM         (1 << XFEATURE_Hi16_ZMM)
> > >  #define XFEATURE_MASK_PT               (1 <<
> > > XFEATURE_PT_UNIMPLEMENTED_SO_FAR)
> > >  #define XFEATURE_MASK_PKRU             (1 << XFEATURE_PKRU)
> > > +#define XFEATURE_MASK_SHSTK_USER       (1 << XFEATURE_SHSTK_USER)
> > > +#define XFEATURE_MASK_SHSTK_KERNEL     (1 << XFEATURE_SHSTK_KERNEL)
> > >
> > >  #define XFEATURE_MASK_FPSSE            (XFEATURE_MASK_FP |
> > > XFEATURE_MASK_SSE)
> > >  #define XFEATURE_MASK_AVX512           (XFEATURE_MASK_OPMASK \
> > > @@ -229,6 +234,23 @@ struct pkru_state {
> > >         u32                             pad;
> > >  } __packed;
> > >
> > > +/*
> > > + * State component 11 is Control flow Enforcement user states
> > > + */
> > > +struct cet_user_state {
> > > +       u64 u_cet;      /* user control flow settings */
> > > +       u64 user_ssp;   /* user shadow stack pointer */
> > > +} __packed;
> > > +
> > > +/*
> > > + * State component 12 is Control flow Enforcement kernel states
> > > + */
> > > +struct cet_kernel_state {
> > > +       u64 kernel_ssp; /* kernel shadow stack */
> > > +       u64 pl1_ssp;    /* ring-1 shadow stack */
> > > +       u64 pl2_ssp;    /* ring-2 shadow stack */
> > > +} __packed;
> > > +
> >
> > Why are these __packed?  It seems like it'll generate bad code for no
> > obvious purpose.
>
> That prevents any possibility that the compiler will insert padding, although in
> 64-bit kernel this should not happen to either struct.  Also all xstate
> components here are packed.
>

They both seem like bugs, perhaps.  As I understand it, __packed
removes padding, but it also forces the compiler to expect the fields
to be unaligned even if they are actually aligned.