Re: [PATCH v5 04/27] x86/fpu/xstate: Add XSAVES system states for shadow stack

[Andy Lutomirski]

On Thu, Nov 8, 2018 at 1:06 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx>
> > > ---
> > >  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.