Subject: Re: [RFC v6 21/62] powerpc: introduce execute-only pkey

From: Thiago Jung Bauermann <[email-masked]>
Date: Fri, 28 Jul 2017 19:17:13 -0300

Ram Pai <[email-masked]> writes:
> --- a/arch/powerpc/mm/pkeys.c
> +++ b/arch/powerpc/mm/pkeys.c
> @@ -97,3 +97,60 @@ int __arch_set_user_pkey_access(struct task_struct *tsk, int pkey,
>  	init_iamr(pkey, new_iamr_bits);
>  	return 0;
>  }
> +
> +static inline bool pkey_allows_readwrite(int pkey)
> +{
> +	int pkey_shift = pkeyshift(pkey);
> +
> +	if (!(read_uamor() & (0x3UL << pkey_shift)))
> +		return true;
> +
> +	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;
> +}

If you follow the code flow in __execute_only_pkey, the AMR and UAMOR
are read 3 times in total, and AMR is written twice. IAMR is read and
written twice. Since they are SPRs and access to them is slow (or isn't
it?), is it worth it to read them once in __execute_only_pkey and pass
down their values to the callees, and then write them once at the end of
the function?

This function is used both by the mmap syscall and the mprotect syscall
(but not by pkey_mprotect) if the requested protection is execute-only.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center