linux-kernel - Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

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Message-ID: <87lgf1v9di.fsf@concordia.ellerman.id.au>
Date:   Fri, 09 Mar 2018 21:19:53 +1100
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Ram Pai <linuxram@...ibm.com>, mingo@...hat.com,
        akpm@...ux-foundation.org
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org, x86@...nel.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        dave.hansen@...el.com, benh@...nel.crashing.org, paulus@...ba.org,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, hbabu@...ibm.com, mhocko@...nel.org,
        bauerman@...ux.vnet.ibm.com, ebiederm@...ssion.com,
        linuxram@...ibm.com, corbet@....net, arnd@...db.de,
        fweimer@...hat.com, msuchanek@...e.com, Ulrich.Weigand@...ibm.com
Subject: Re: [PATCH] x86, powerpc : pkey-mprotect must allow pkey-0

Ram Pai <linuxram@...ibm.com> writes:

> Once an address range is associated with an allocated pkey, it cannot be
> reverted back to key-0. There is no valid reason for the above behavior.  On
> the contrary applications need the ability to do so.

Please explain this in much more detail. Is it an ABI change?

And why did we just notice this?

> The patch relaxes the restriction.
>
> Tested on powerpc and x86_64.

Thanks, but please split the patch, one for each arch.

cheers

> diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
> index 0409c80..3e8abe4 100644
> --- a/arch/powerpc/include/asm/pkeys.h
> +++ b/arch/powerpc/include/asm/pkeys.h
> @@ -101,10 +101,18 @@ static inline u16 pte_to_pkey_bits(u64 pteflags)
>  
>  static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
>  {
> -	/* A reserved key is never considered as 'explicitly allocated' */
> -	return ((pkey < arch_max_pkey()) &&
> -		!__mm_pkey_is_reserved(pkey) &&
> -		__mm_pkey_is_allocated(mm, pkey));
> +	/* pkey 0 is allocated by default. */
> +	if (!pkey)
> +	       return true;
> +
> +	if (pkey < 0 || pkey >= arch_max_pkey())
> +	       return false;
> +
> +	/* reserved keys are never allocated. */
> +	if (__mm_pkey_is_reserved(pkey))
> +	       return false;
> +
> +	return(__mm_pkey_is_allocated(mm, pkey));
>  }
>  
>  extern void __arch_activate_pkey(int pkey);
> @@ -150,7 +158,8 @@ static inline int mm_pkey_free(struct mm_struct *mm, int pkey)
>  	if (static_branch_likely(&pkey_disabled))
>  		return -1;
>  
> -	if (!mm_pkey_is_allocated(mm, pkey))
> +	/* pkey 0 cannot be freed */
> +	if (!pkey || !mm_pkey_is_allocated(mm, pkey))
>  		return -EINVAL;
>  
>  	/*
> diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
> index a0ba1ff..6ea7486 100644
> --- a/arch/x86/include/asm/pkeys.h
> +++ b/arch/x86/include/asm/pkeys.h
> @@ -52,7 +52,7 @@ bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
>  	 * from pkey_alloc().  pkey 0 is special, and never
>  	 * returned from pkey_alloc().
>  	 */
> -	if (pkey <= 0)
> +	if (pkey < 0)
>  		return false;
>  	if (pkey >= arch_max_pkey())
>  		return false;
> @@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)
>  static inline
>  int mm_pkey_free(struct mm_struct *mm, int pkey)
>  {
> -	if (!mm_pkey_is_allocated(mm, pkey))
> +	/* pkey 0 is special and can never be freed */
> +	if (!pkey || !mm_pkey_is_allocated(mm, pkey))
>  		return -EINVAL;
>  
>  	mm_set_pkey_free(mm, pkey);
> -- 
> 1.8.3.1