[PATCH 09/25] powerpc: ability to create execute-disabled pkeys

[Ram Pai]

On Wed, Oct 18, 2017 at 02:42:56PM +1100, Balbir Singh wrote:
> On Fri,  8 Sep 2017 15:44:57 -0700
> Ram Pai <linuxram at us.ibm.com> wrote:
> 
> > powerpc has hardware support to disable execute on a pkey.
> > This patch enables the ability to create execute-disabled
> > keys.
> > 
> > Signed-off-by: Ram Pai <linuxram at us.ibm.com>
> > ---
> >  arch/powerpc/include/uapi/asm/mman.h |    6 ++++++
> >  arch/powerpc/mm/pkeys.c              |   16 ++++++++++++++++
> >  2 files changed, 22 insertions(+), 0 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/uapi/asm/mman.h b/arch/powerpc/include/uapi/asm/mman.h
> > index ab45cc2..f272b09 100644
> > --- a/arch/powerpc/include/uapi/asm/mman.h
> > +++ b/arch/powerpc/include/uapi/asm/mman.h
> > @@ -45,4 +45,10 @@
> >  #define MAP_HUGE_1GB	(30 << MAP_HUGE_SHIFT)	/* 1GB   HugeTLB Page */
> >  #define MAP_HUGE_16GB	(34 << MAP_HUGE_SHIFT)	/* 16GB  HugeTLB Page */
> >  
> > +/* override any generic PKEY Permission defines */
> > +#define PKEY_DISABLE_EXECUTE   0x4
> > +#undef PKEY_ACCESS_MASK
> > +#define PKEY_ACCESS_MASK       (PKEY_DISABLE_ACCESS |\
> > +				PKEY_DISABLE_WRITE  |\
> > +				PKEY_DISABLE_EXECUTE)
> >  #endif /* _UAPI_ASM_POWERPC_MMAN_H */
> > diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
> > index cc5be6a..2282864 100644
> > --- a/arch/powerpc/mm/pkeys.c
> > +++ b/arch/powerpc/mm/pkeys.c
> > @@ -24,6 +24,14 @@ void __init pkey_initialize(void)
> >  {
> >  	int os_reserved, i;
> >  
> > +	/*
> > +	 * we define PKEY_DISABLE_EXECUTE in addition to the arch-neutral
> > +	 * generic defines for PKEY_DISABLE_ACCESS and PKEY_DISABLE_WRITE.
> > +	 * Ensure that the bits a distinct.
> > +	 */
> > +	BUILD_BUG_ON(PKEY_DISABLE_EXECUTE &
> > +		     (PKEY_DISABLE_ACCESS | PKEY_DISABLE_WRITE));
> 
> Will these values every change? It's good to have I guess.
> 
> > +
> >  	/* disable the pkey system till everything
> >  	 * is in place. A patch further down the
> >  	 * line will enable it.
> > @@ -120,10 +128,18 @@ int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
> >  		unsigned long init_val)
> >  {
> >  	u64 new_amr_bits = 0x0ul;
> > +	u64 new_iamr_bits = 0x0ul;
> >  
> >  	if (!is_pkey_enabled(pkey))
> >  		return -EINVAL;
> >  
> > +	if ((init_val & PKEY_DISABLE_EXECUTE)) {
> > +		if (!pkey_execute_disable_support)
> > +			return -EINVAL;
> > +		new_iamr_bits |= IAMR_EX_BIT;
> > +	}
> > +	init_iamr(pkey, new_iamr_bits);
> > +
> 
> Where do we check the reserved keys?

The main gate keeper against spurious keys are the system calls.
sys_pkey_mprotect(), sys_pkey_free() and sys_pkey_modify() are the one
that will check against reserved and unallocated keys.  Once it has
passed the check, all other internal functions trust the key values
provided to them. I can put in additional checks but that will
unnecessarily chew a few cpu cycles.

Agree?

BTW: you raise a good point though, I may have missed guarding against
unallocated or reserved keys in sys_pkey_modify(). That was a power
specific system call that I have introduced to change the permissions on
a key.

RP