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

[Balbir Singh]

On Fri,  8 Sep 2017 15:44:59 -0700
Ram Pai <linuxram at 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 at 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.

> +
> +	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 at gmail.com>