linux-hardening - Re: [RFC PATCH 02/13] powerpc: Add initial Dynamic Execution Control Register (DEXCR) support

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Date:   Fri, 10 Mar 2023 10:46:51 +1100
From:   Benjamin Gray <bgray@...ux.ibm.com>
To:     Nicholas Piggin <npiggin@...il.com>, linuxppc-dev@...ts.ozlabs.org
Cc:     linux-hardening@...r.kernel.org, ajd@...ux.ibm.com,
        cmr@...escreens.de, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 02/13] powerpc: Add initial Dynamic Execution
 Control Register (DEXCR) support

On Tue, 2023-03-07 at 14:45 +1000, Nicholas Piggin wrote:
> On Mon Nov 28, 2022 at 12:44 PM AEST, Benjamin Gray wrote:
> > diff --git a/arch/powerpc/include/asm/cputable.h
> > b/arch/powerpc/include/asm/cputable.h
> > index 757dbded11dc..03bc192f2d8b 100644
> > --- a/arch/powerpc/include/asm/cputable.h
> > +++ b/arch/powerpc/include/asm/cputable.h
> > @@ -192,6 +192,10 @@ static inline void cpu_feature_keys_init(void)
> > { }
> >  #define
> > CPU_FTR_P9_RADIX_PREFETCH_BUG  LONG_ASM_CONST(0x0002000000000000)
> >  #define
> > CPU_FTR_ARCH_31                        LONG_ASM_CONST(0x00040000000
> > 00000)
> >  #define
> > CPU_FTR_DAWR1                  LONG_ASM_CONST(0x0008000000000000)
> > +#define
> > CPU_FTR_DEXCR_SBHE             LONG_ASM_CONST(0x0010000000000000)
> > +#define
> > CPU_FTR_DEXCR_IBRTPD           LONG_ASM_CONST(0x0020000000000000)
> > +#define
> > CPU_FTR_DEXCR_SRAPD            LONG_ASM_CONST(0x0040000000000000)
> > +#define
> > CPU_FTR_DEXCR_NPHIE            LONG_ASM_CONST(0x0080000000000000)
> 
> We potentially don't need to use CPU_FTR bits for each of these. We
> only really want them to use instruction patching and make feature
> tests fast. But we have been a bit liberal with using them and they
> are kind of tied into cpu feature parsing code so maybe it's easier
> to go with them for now.

For the static only DEXCR series I've only got CPU_FTR_DEXCR_NPHIE
because that's needed for hashkey updates. The others don't really
matter; they are only interesting for masking out unsupported bits.
Masking itself seems to be unnecessary; the DEXCR will just ignore
unsupported bits. Attempting to set all bits on a P10 showed the first
8 were set and the remainder stayed 0'd, and the kernel worked fine.

It's definitely easier to use CPU_FTR_* for feature detection from the
PAPR specified blob though. Maybe it would be possible to support a
callback on a match instead of setting a feature flag.
@@ -1802,7 +1809,7 @@ int copy_thread(struct task_struct *p, const
> 
> 

> > @@ -1802,7 +1809,7 @@ int copy_thread(struct task_struct *p, const
> > struct kernel_clone_args *args)
> >  
> >         setup_ksp_vsid(p, sp);
> >  
> > -#ifdef CONFIG_PPC64 
> > +#ifdef CONFIG_PPC64
> >         if (cpu_has_feature(CPU_FTR_DSCR)) {
> >                 p->thread.dscr_inherit = current-
> > >thread.dscr_inherit;
> >                 p->thread.dscr = mfspr(SPRN_DSCR);
> > @@ -1939,6 +1946,10 @@ void start_thread(struct pt_regs *regs,
> > unsigned long start, unsigned long sp)
> >         current->thread.tm_tfiar = 0;
> >         current->thread.load_tm = 0;
> >  #endif /* CONFIG_PPC_TRANSACTIONAL_MEM */
> > +#ifdef CONFIG_PPC_BOOK3S_64
> > +       if (cpu_has_feature(CPU_FTR_ARCH_31))
> > +               mtspr(SPRN_DEXCR, get_thread_dexcr(&current-
> > >thread));
> > +#endif /* CONFIG_PPC_BOOK3S_64 */
> 
> You possibly don't need the ifdef here because CPU_FTR_ARCH_31 should
> fold away. Some of the others do because they're using open-coded
> access to struct members, but if you're using accessor functions to
> get and set such things, there may be no need to.
> 
> I think my preference is for your style.

I've been revisiting where the DEXCR is initialised and updated. With
the static DEXCR, the thread value is just a field on the task struct
like the others.