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

Message ID 1504910713-7094-20-git-send-email-linuxram@us.ibm.com
State Changes Requested
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Series
  • powerpc: Free up RPAGE_RSV bits
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Commit Message

Ram Pai Sept. 8, 2017, 10:44 p.m.
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(-)

Comments

Balbir Singh Oct. 18, 2017, 4:15 a.m. | #1
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.

> +
> +	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>
Ram Pai Oct. 18, 2017, 8:57 p.m. | #2
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.
Balbir Singh Oct. 18, 2017, 11:02 p.m. | #3
On Wed, 18 Oct 2017 13:57:39 -0700
Ram Pai <linuxram@us.ibm.com> wrote:

> 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.
>

I agree, thats why I said it's a bug if the values are such.
Do we care to validate that both bits are the same?

Balbir Singh.
Ram Pai Oct. 19, 2017, 3:52 p.m. | #4
On Thu, Oct 19, 2017 at 10:02:13AM +1100, Balbir Singh wrote:
> On Wed, 18 Oct 2017 13:57:39 -0700
> Ram Pai <linuxram@us.ibm.com> wrote:
> 
> > 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.
> >
> 
> I agree, thats why I said it's a bug if the values are such.
> Do we care to validate that both bits are the same?

I will put in a assert. Will that work?

RP

> 
> Balbir Singh.

Patch

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;
+
+	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;
+}