Re: [Xen-devel] [v3 04/12] x86/fsgsbase/64: Enable FSGSBASE instructions in the helper functions

[Andy Lutomirski]

On Thu, Oct 25, 2018 at 4:09 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 25/10/2018 07:09, Juergen Gross wrote:
> > On 24/10/2018 21:41, Andrew Cooper wrote:
> >> On 24/10/18 20:16, Andy Lutomirski wrote:
> >>> On Tue, Oct 23, 2018 at 11:43 AM Chang S. Bae <chang.seok.bae@xxxxxxxxx> wrote:
> >>>> The helper functions will switch on faster accesses to FSBASE and GSBASE
> >>>> when the FSGSBASE feature is enabled.
> >>>>
> >>>> Accessing user GSBASE needs a couple of SWAPGS operations. It is avoidable
> >>>> if the user GSBASE is saved at kernel entry, being updated as changes, and
> >>>> restored back at kernel exit. However, it seems to spend more cycles for
> >>>> savings and restorations. Little or no benefit was measured from
> >>>> experiments.
> >>>>
> >>>> Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
> >>>> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> >>>> Cc: Any Lutomirski <luto@xxxxxxxxxx>
> >>>> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> >>>> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >>>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> >>>> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >>>> ---
> >>>>  arch/x86/include/asm/fsgsbase.h | 17 +++----
> >>>>  arch/x86/kernel/process_64.c    | 82 +++++++++++++++++++++++++++------
> >>>>  2 files changed, 75 insertions(+), 24 deletions(-)
> >>>>
> >>>> diff --git a/arch/x86/include/asm/fsgsbase.h b/arch/x86/include/asm/fsgsbase.h
> >>>> index b4d4509b786c..e500d771155f 100644
> >>>> --- a/arch/x86/include/asm/fsgsbase.h
> >>>> +++ b/arch/x86/include/asm/fsgsbase.h
> >>>> @@ -57,26 +57,23 @@ static __always_inline void wrgsbase(unsigned long gsbase)
> >>>>                         : "memory");
> >>>>  }
> >>>>
> >>>> +#include <asm/cpufeature.h>
> >>>> +
> >>>>  /* Helper functions for reading/writing FS/GS base */
> >>>>
> >>>>  static inline unsigned long x86_fsbase_read_cpu(void)
> >>>>  {
> >>>>         unsigned long fsbase;
> >>>>
> >>>> -       rdmsrl(MSR_FS_BASE, fsbase);
> >>>> +       if (static_cpu_has(X86_FEATURE_FSGSBASE))
> >>>> +               fsbase = rdfsbase();
> >>>> +       else
> >>>> +               rdmsrl(MSR_FS_BASE, fsbase);
> >>>>
> >>>>         return fsbase;
> >>>>  }
> >>>>
> >>>> -static inline unsigned long x86_gsbase_read_cpu_inactive(void)
> >>>> -{
> >>>> -       unsigned long gsbase;
> >>>> -
> >>>> -       rdmsrl(MSR_KERNEL_GS_BASE, gsbase);
> >>>> -
> >>>> -       return gsbase;
> >>>> -}
> >>>> -
> >>>> +extern unsigned long x86_gsbase_read_cpu_inactive(void);
> >>>>  extern void x86_fsbase_write_cpu(unsigned long fsbase);
> >>>>  extern void x86_gsbase_write_cpu_inactive(unsigned long gsbase);
> >>>>
> >>>> diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
> >>>> index 31b4755369f0..fcf18046c3d6 100644
> >>>> --- a/arch/x86/kernel/process_64.c
> >>>> +++ b/arch/x86/kernel/process_64.c
> >>>> @@ -159,6 +159,36 @@ enum which_selector {
> >>>>         GS
> >>>>  };
> >>>>
> >>>> +/*
> >>>> + * Interrupts are disabled here. Out of line to be protected from kprobes.
> >>>> + */
> >>>> +static noinline __kprobes unsigned long rd_inactive_gsbase(void)
> >>>> +{
> >>>> +       unsigned long gsbase, flags;
> >>>> +
> >>>> +       local_irq_save(flags);
> >>>> +       native_swapgs();
> >>>> +       gsbase = rdgsbase();
> >>>> +       native_swapgs();
> >>>> +       local_irq_restore(flags);
> >>>> +
> >>>> +       return gsbase;
> >>>> +}
> >>> Please fold this into its only caller and make *that* noinline.
> >>>
> >>> Also, this function, and its "write" equivalent, will access the
> >>> *active* gsbase.  So it either needs to be fixed for Xen PV or some
> >>> clear comment and careful auditing needs to be added to ensure that
> >>> it's not used on Xen PV.  Or it needs to be renamed
> >>> native_x86_fsgsbase_... and add paravirt hooks, since Xen PV allows a
> >>> very efficient but different implementation, I think.  The latter is
> >>> probably the right solution.
> >>>
> >>> (Hi Xen people -- how does CR4.FSGSBASE work on Xen?  Is it always
> >>> set?  Never set?  Set only if the guest tries to set it?)
> >> FML.  Seriously - whoever put this code into the hypervisor in the past
> >> did an atrocious job.  After some experimentation, you're going to be
> >> sad and I'm declaring this borderline unusable.
> >>
> >> Looks like Xen unconditionally enabled CR4.FSGSBASE if it is available.
> >> Therefore, PV guests can use the instructions, even if the bit is clear
> >> in vCR4.
> >>
> >> The CPUID bits are exposed to guests by default, and Xen will emulate
> >> vCR4.FSGSBASE being set and cleared.
> >>
> >> We don't however emulate swapgs (which is a cpl0 instruction).  The
> >> guest gets handed a #GP[0] instead.
> >>
> >> The Linux WRMSR PVop uses the set_segment_base() hypercall in instead of
> >> going through the full wrmsr emulation path.
> >>
> >> There is no equivalent get hypercall, so the only way I can see of
> >> getting the value is to actually read MSR_KERNEL_GS_BASE and take the
> >> full rdmsr emulation path.
> > Or shadow the value in a percpu variable.
>
> Hmm true, so long as no paths try to use native_rd{fs,gs}base() to
> bypass the PVop.

But *user* code can change the base.  How is the kernel supposed to
context-switch the user gsbase?