Re: [PATCH 11/25] powerpc: introduce execute-only pkey

[Ram Pai <linuxram@us.ibm.com>]

On Wed, Oct 18, 2017 at 03:15:22PM +1100, Balbir Singh wrote:
> On Fri,  8 Sep 2017 15:44:59 -0700
> Ram Pai <linuxram@us.ibm.com> wrote:
> 
> > This patch provides the implementation of execute-only pkey.
> > The architecture-independent layer expects the arch-dependent
> > layer, to support the ability to create and enable a special
> > key which has execute-only permission.
> > 
> > Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> > ---
> >  arch/powerpc/include/asm/book3s/64/mmu.h |    1 +
> >  arch/powerpc/include/asm/pkeys.h         |    9 ++++-
> >  arch/powerpc/mm/pkeys.c                  |   57 ++++++++++++++++++++++++++++++
> >  3 files changed, 66 insertions(+), 1 deletions(-)
> > 
> > diff --git a/arch/powerpc/include/asm/book3s/64/mmu.h b/arch/powerpc/include/asm/book3s/64/mmu.h
> > index 55950f4..ee18ba0 100644
> > --- a/arch/powerpc/include/asm/book3s/64/mmu.h
> > +++ b/arch/powerpc/include/asm/book3s/64/mmu.h
> > @@ -115,6 +115,7 @@ struct patb_entry {
> >  	 * bit unset -> key available for allocation
> >  	 */
> >  	u32 pkey_allocation_map;
> > +	s16 execute_only_pkey; /* key holding execute-only protection */
> >  #endif
> >  } mm_context_t;
> >  
> > diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
> > index 78c5362..0cf115f 100644
> > --- a/arch/powerpc/include/asm/pkeys.h
> > +++ b/arch/powerpc/include/asm/pkeys.h
> > @@ -115,11 +115,16 @@ static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
> >   * Try to dedicate one of the protection keys to be used as an
> >   * execute-only protection key.
> >   */
> > +extern int __execute_only_pkey(struct mm_struct *mm);
> >  static inline int execute_only_pkey(struct mm_struct *mm)
> >  {
> > -	return 0;
> > +	if (!pkey_inited || !pkey_execute_disable_support)
> > +		return -1;
> > +
> > +	return __execute_only_pkey(mm);
> >  }
> >  
> > +
> >  static inline int arch_override_mprotect_pkey(struct vm_area_struct *vma,
> >  		int prot, int pkey)
> >  {
> > @@ -141,6 +146,8 @@ static inline void pkey_mm_init(struct mm_struct *mm)
> >  	if (!pkey_inited)
> >  		return;
> >  	mm_pkey_allocation_map(mm) = initial_allocation_mask;
> > +	/* -1 means unallocated or invalid */
> > +	mm->context.execute_only_pkey = -1;
> >  }
> >  
> >  extern void thread_pkey_regs_save(struct thread_struct *thread);
> > diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
> > index 7cd1be4..8a24983 100644
> > --- a/arch/powerpc/mm/pkeys.c
> > +++ b/arch/powerpc/mm/pkeys.c
> > @@ -188,3 +188,60 @@ void thread_pkey_regs_init(struct thread_struct *thread)
> >  	write_iamr(0x0ul);
> >  	write_uamor(0x0ul);
> >  }
> > +
> > +static inline bool pkey_allows_readwrite(int pkey)
> > +{
> > +	int pkey_shift = pkeyshift(pkey);
> > +
> > +	if (!(read_uamor() & (0x3UL << pkey_shift)))
> > +		return true;
> 
> If uamor for key 0 is 0x10 for example or 0x01 it's a bug.
> The above check might miss it.


The specs says both the bits corresponding to a key are set or
reset, cannot be anything else.

cut-n-paste from the ISA...
----------------------------------------------------
Software must ensure that both bits of each even/odd
bit pair of the AMOR contain the same value. -- i.e.,
the contents of register RS for mtspr specifying the
AMOR must be such that (RS)2n = (RS)2n+1 for every
n in the range 0:31 - and like for the UAMOR.
---------------------------------------------------------

> 
> > +
> > +	return !(read_amr() & ((AMR_RD_BIT|AMR_WR_BIT) << pkey_shift));
> > +}
> > +
> > +int __execute_only_pkey(struct mm_struct *mm)
> > +{
> > +	bool need_to_set_mm_pkey = false;
> > +	int execute_only_pkey = mm->context.execute_only_pkey;
> > +	int ret;
> > +
> > +	/* Do we need to assign a pkey for mm's execute-only maps? */
> > +	if (execute_only_pkey == -1) {
> > +		/* Go allocate one to use, which might fail */
> > +		execute_only_pkey = mm_pkey_alloc(mm);
> > +		if (execute_only_pkey < 0)
> > +			return -1;
> > +		need_to_set_mm_pkey = true;
> > +	}
> > +
> > +	/*
> > +	 * We do not want to go through the relatively costly
> > +	 * dance to set AMR if we do not need to.  Check it
> > +	 * first and assume that if the execute-only pkey is
> > +	 * readwrite-disabled than we do not have to set it
> > +	 * ourselves.
> > +	 */
> > +	if (!need_to_set_mm_pkey &&
> > +	    !pkey_allows_readwrite(execute_only_pkey))
> > +		return execute_only_pkey;
> > +
> > +	/*
> > +	 * Set up AMR so that it denies access for everything
> > +	 * other than execution.
> > +	 */
> > +	ret = __arch_set_user_pkey_access(current, execute_only_pkey,
> > +			(PKEY_DISABLE_ACCESS | PKEY_DISABLE_WRITE));
> > +	/*
> > +	 * If the AMR-set operation failed somehow, just return
> > +	 * 0 and effectively disable execute-only support.
> > +	 */
> > +	if (ret) {
> > +		mm_set_pkey_free(mm, execute_only_pkey);
> > +		return -1;
> > +	}
> > +
> > +	/* We got one, store it and use it from here on out */
> > +	if (need_to_set_mm_pkey)
> > +		mm->context.execute_only_pkey = execute_only_pkey;
> > +	return execute_only_pkey;
> > +}
> 
> Looks good otherwise
> 
> Acked-by: Balbir Singh <bsingharora@gmail.com>

thanks.

-- 
Ram Pai