linux-kernel - Re: [RFC v6 27/62] powerpc: helper to validate key-access permissions of a pte

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Date:   Fri, 28 Jul 2017 18:00:02 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
        arnd@...db.de, corbet@....net, mhocko@...nel.org,
        dave.hansen@...el.com, mingo@...hat.com, paulus@...ba.org,
        aneesh.kumar@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
        khandual@...ux.vnet.ibm.com
Subject: Re: [RFC v6 27/62] powerpc: helper to validate key-access permissions of a pte


Ram Pai <linuxram@...ibm.com> writes:
> --- a/arch/powerpc/mm/pkeys.c
> +++ b/arch/powerpc/mm/pkeys.c
> @@ -201,3 +201,36 @@ int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot,
>  	 */
>  	return vma_pkey(vma);
>  }
> +
> +static bool pkey_access_permitted(int pkey, bool write, bool execute)
> +{
> +	int pkey_shift;
> +	u64 amr;
> +
> +	if (!pkey)
> +		return true;
> +
> +	pkey_shift = pkeyshift(pkey);
> +	if (!(read_uamor() & (0x3UL << pkey_shift)))
> +		return true;
> +
> +	if (execute && !(read_iamr() & (IAMR_EX_BIT << pkey_shift)))
> +		return true;
> +
> +	if (!write) {
> +		amr = read_amr();
> +		if (!(amr & (AMR_RD_BIT << pkey_shift)))
> +			return true;
> +	}
> +
> +	amr = read_amr(); /* delay reading amr uptil absolutely needed */

Actually, this is causing amr to be read twice in case control enters
the "if (!write)" block above but doesn't enter the other if block nested
in it.

read_amr should be called only once, right before "if (!write)".

-- 
Thiago Jung Bauermann
IBM Linux Technology Center