powerpc, pkey: make protection key 0 less special

arch/powerpc/mm/pkeys.c static bool pkey_access_permitted(int pkey, bool write, bool execute) · line 418

Quoted 9 lines · Ram Pai

arch/powerpc/mm/pkeys.c +0 -3

@@ -407,9 +414,6 @@ static bool pkey_access_permitted(int pkey, bool write, bool execute)
 	int pkey_shift;
 	u64 amr;
 
-	if (!pkey)
-		return true;
-
 	if (!is_pkey_enabled(pkey))
 		return true;

Looks fine to me. Obviously doesn't have any impact on x86 or the generic code.

One question, though. Which other check makes up for this removed !pkey check?

Applications need the ability to associate an address-range with some key and latter revert to its initial default key. Pkey-0 comes close to providing this function but falls short, because the current implementation disallows applications to explicitly associate pkey-0 to the address range.

Lets make pkey-0 less special and treat it almost like any other key. Thus it can be explicitly associated with any address range, and can be freed. This gives the application more flexibility and power. The ability to free pkey-0 must be used responsibily, since pkey-0 is associated with almost all address-range by default.

Even with this change pkey-0 continues to be slightly more special from the following point of view.
(a) it is implicitly allocated.
(b) it is the default key assigned to any address-range.
(c) its permissions cannot be modified by userspace.

NOTE: (c) is specific to powerpc only. pkey-0 is associated by default with all pages including kernel pages, and pkeys are also active in kernel mode. If any permission is denied on pkey-0, the kernel running in the context of the application will be unable to operate.

Tested on powerpc.

cc Thomas Gleixner <[email protected]>

cc Dave Hansen <[email protected]>

cc Michael Ellermen <[email protected]>

cc Ingo Molnar <[email protected]>

cc Andrew Morton <[email protected]>

Signed-off-by Ram Pai <[email protected]>

---
History:

v3: . Corrected a comment in arch_set_user_pkey_access().
. Clarified the header, to capture the notion that
pkey-0 permissions cannot be modified by userspace on
powerpc. -- comment from Thiago

v2: . mm_pkey_is_allocated() continued to treat pkey-0
special. fixed it.

 arch/powerpc/include/asm/pkeys.h |   22 ++++++++++++++++++----
 arch/powerpc/mm/pkeys.c          |   26 +++++++++++++++-----------
 2 files changed, 33 insertions(+), 15 deletions(-)

(quoted diff) +0 -0

b/arch/powerpc/include/asm/pkeys.h index 0409c80..31a6976 100644

arch/powerpc/include/asm/pkeys.h +0 -1

@@ -101,10 +101,14 @@ 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' */

arch/powerpc/include/asm/pkeys.h +18 -3

-	return ((pkey < arch_max_pkey()) &&
-		!__mm_pkey_is_reserved(pkey) &&
-		__mm_pkey_is_allocated(mm, pkey));
+	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);

@@ -200,6 +204,16 @@ static inline int
 arch_set_user_pkey_access(struct task_struct *tsk, int pkey, {
 	if (static_branch_likely(&pkey_disabled))
 		return -EINVAL;
+
+	/*
+	 * userspace should not change pkey-0 permissions.
+	 * pkey-0 is associated with every page in the kernel.
+	 * If userspace denies any permission on pkey-0, the
+	 * kernel cannot operate.
+	 */
+	if (!pkey)
+		return init_val ? -EINVAL : 0;
+
 	return __arch_set_user_pkey_access(tsk, pkey, init_val);
 }
 

arch/powerpc/mm/pkeys.c +12 -7

index 0eafdf0..d6873b4 100644
@@ -119,16 +119,21 @@ int pkey_initialize(void)
 #else
 	os_reserved = 0;
 #endif
+	/* Bits are in LE format. */
 	initial_allocation_mask = ~0x0;
+
+	/* register mask is in BE format */
 	pkey_amr_uamor_mask = ~0x0ul;
 	pkey_iamr_mask = ~0x0ul;
-	/*
-	 * key 0, 1 are reserved.
-	 * key 0 is the default key, which allows read/write/execute.
-	 * key 1 is recommended not to be used. PowerISA(3.0) page
 1015,
-	 * programming note.
-	 */
-	for (i = 2; i < (pkeys_total - os_reserved); i++) {
+
+	for (i = 0; i < (pkeys_total - os_reserved); i++) {
+		/*
+		 * key 1 is recommended not to be used.
+		 * PowerISA(3.0) page 1015,
+		 */
+		if (i == 1)
+			continue;
+
 		initial_allocation_mask &= ~(0x1 << i);
 		pkey_amr_uamor_mask &= ~(0x3ul << pkeyshift(i));

arch/powerpc/mm/pkeys.c +3 -4

 		pkey_iamr_mask &= ~(0x1ul << pkeyshift(i));

@@ -142,7 +147,9 @@ void pkey_mm_init(struct mm_struct *mm)
 {
 	if (static_branch_likely(&pkey_disabled))
 		return;
-	mm_pkey_allocation_map(mm) = initial_allocation_mask;
+
+	/* allocate key-0 by default */
+	mm_pkey_allocation_map(mm) = initial_allocation_mask | 0x1;
 	/* -1 means unallocated or invalid */
 	mm->context.execute_only_pkey = -1;
 }

@@ -407,9 +414,6 @@ static bool pkey_access_permitted(int pkey, bool
 write, bool execute) int pkey_shift;
 	u64 amr;
 
-	if (!pkey)
-		return true;
-
 	if (!is_pkey_enabled(pkey))
 		return true;
 

If it is not ok to change permissions of pkey 0 is it ok to free it?

Quoted 1 line · Michal Suchánek

If it is not ok to change permissions of pkey 0 is it ok to free it?

It's pretty much never OK to free it on x86 or ppc. But, we're not going to put code in to keep userspace from shooting itself in the foot, at least on x86.

Quoted 7 lines · Dave Hansen

On 05/04/2018 02:26 PM, Michal Such=C3=A1nek wrote: =20

Quoted 2 lines

If it is not ok to change permissions of pkey 0 is it ok to free it? =20

=20
It's pretty much never OK to free it on x86 or ppc. But, we're not going to put code in to keep userspace from shooting itself in the foot, at least on x86. =20

=20
and on powerpc aswell.

Quoted 5 lines · On Fri, 4 May 2018 14:45:07 -0700 Ram Pai <[email protected]> wrote:

Quoted 4 lines · Dave Hansen

Quoted 2 lines · Michal Suchánek

If it is not ok to change permissions of pkey 0 is it ok to free it?

Quoted 5 lines

Quoted 3 lines

It's pretty much never OK to free it on x86 or ppc. But, we're not going to put code in to keep userspace from shooting itself in the foot, at least on x86.

and on powerpc aswell.

But once it's free it can be re-allocated. So you are moving the special-casing from free code to code dealing with allocation.

If you want something like allocate_exec_only_pkey then the function (either in kernel or in userspace) needs to make sure it is not getting/requesting key 0 on powerpc.

Thanks

Michal

Quoted 11 lines · On Sat, May 05, 2018 at 02:39:56PM +0200, Michal Such=C3=A1nek wrote:

On Fri, 4 May 2018 14:45:07 -0700
Ram Pai <[email protected]> wrote:
=20

Quoted 8 lines

On Fri, May 04, 2018 at 02:31:10PM -0700, Dave Hansen wrote: =20

Quoted 7 lines

On 05/04/2018 02:26 PM, Michal Such=C3=A1nek wrote: =20

Quoted 2 lines

If it is not ok to change permissions of pkey 0 is it ok to
free it? =20

=20
It's pretty much never OK to free it on x86 or ppc. But, we're not going to put code in to keep userspace from shooting itself
in the foot, at least on x86. =20

Quoted 5 lines

Quoted 2 lines

=20
and on powerpc aswell. =20

=20
But once it's free it can be re-allocated. So you are moving the special-casing from free code to code dealing with allocation. =20

=20
Actually if an application frees key-0, it has potentially opened up a can-of-worms. It could step on anything that explodes.=20 =20
Its choice between imposing policies on an application v/s freeing it up to choose its own policy. I think the kernel should impose some form of mild-policy. But others think there should be none.

=20

Quoted 4 lines

=20
If you want something like allocate_exec_only_pkey then the function (either in kernel or in userspace) needs to make sure it is not getting/requesting key 0 on powerpc. =20

=20
Yes. makes sense. I will put in some checks towards that.

Quoted 18 lines · On Sun, 6 May 2018 13:10:43 -0700 Ram Pai <[email protected]> wrote:

Quoted 8 lines · Michal Suchánek

Quoted 5 lines · On Fri, 4 May 2018 14:45:07 -0700 Ram Pai <[email protected]> wrote:

Quoted 4 lines · Dave Hansen

Quoted 2 lines · Michal Suchánek

If it is not ok to change permissions of pkey 0 is it ok to free it?

Quoted 8 lines

Quoted 5 lines

Quoted 3 lines

It's pretty much never OK to free it on x86 or ppc. But, we're not going to put code in to keep userspace from shooting itself in the foot, at least on x86.

and on powerpc aswell.

But once it's free it can be re-allocated. So you are moving the special-casing from free code to code dealing with allocation.

Quoted 6 lines

Actually if an application frees key-0, it has potentially opened up a can-of-worms. It could step on anything that explodes.

Its choice between imposing policies on an application v/s freeing it up to choose its own policy. I think the kernel should impose some form of mild-policy. But others think there should be none.

It is a problem of the application when it frees a key it still needs. I am not for or against allowing that. The problem is with the interface change once the key is freed.

Quoted 7 lines

Quoted 5 lines

If you want something like allocate_exec_only_pkey then the function (either in kernel or in userspace) needs to make sure it is not getting/requesting key 0 on powerpc.

Yes. makes sense. I will put in some checks towards that.

Suppose an application is adapted to take advantage of freeing key 0, perhaps to revoke access to any code and data used at runtime initialization which is not longer needed.

As I understand it with the proposed change any address range not associated with non-default key becomes inaccessible and key 0 becomes available for allocation again.

This is fine on x86 where key 0 is fully functional.

However, on powerpc the application now has key 0 available and it is not fully a fully functional key. So the application now needs to check that the key it is allocating is not key 0. Otherwise further key operations may unexpectedly fail.

To prevent this I would suggest

a) do not allow allocating key 0. If it is freed it becomes reserved

b) never expose key 0 to applications. Allocate key 2 as the default key and present the key numbers with an offset to userspace so key 2 appears as key 0 to user applications.

Thanks

Michal

Quoted 4 lines · On Mon, May 07, 2018 at 01:21:49PM +0200, Michal Such=C3=A1nek wrote:

On Sun, 6 May 2018 13:10:43 -0700
Ram Pai <[email protected]> wrote:
=20

Quoted 1 line

On Sat, May 05, 2018 at 02:39:56PM +0200, Michal Such=C3=A1nek wrote:=

Quoted 2 lines

Quoted 2 lines

Quoted 2 lines

On Fri, 4 May 2018 14:45:07 -0700
Ram Pai <[email protected]> wrote:

Quoted 7 lines

Suppose an application is adapted to take advantage of freeing key 0, perhaps to revoke access to any code and data used at runtime initialization which is not longer needed.=20
=20
As I understand it with the proposed change any address range not associated with non-default key becomes inaccessible and key 0 becomes available for allocation again. =20

=20
The above sentence is difficult to parse. I think what you are saying is -- Any address range associated with key-0 become inaccessible if key-0 is freed. Is that a correct simplification of the above
statement?
=20
Assuming I got it right -- =20
=20
Yes. key-0 when freed becomes available for allocation again. No. When key-0 is freed any address range
associated with key-0 will continue to be accessible. It was accessible to begin with and will continue to be accessible. It was accessible, because kernel; during task-creation, never disables any permissions on key-0, and also never lets userspace disable any permissions on key-0.

=20
The problem arises, when key-0 gets reallocated at a later time. If the key-0 at that point(during reallocation) is treated like any other key; allowing userspace to change its permissions, it can explode on access to any page associated with key-0. (BTW: almost everything is associated with key-0 by default).
=20

Quoted 1 line

This is fine on x86 where key 0 is fully functional. =20

=20

Quoted 10 lines

=20
However, on powerpc the application now has key 0 available and it is not fully a fully functional key. So the application now needs to check that the key it is allocating is not key 0. Otherwise further key operations may unexpectedly fail.
=20
To prevent this I would suggest
=20
a) do not allow allocating key 0. If it is freed it becomes reserved =20

=20
Certainly yes. We never promised any particular key to the user space. So not returning key-0 during allocation should break no semantics.
=20

Quoted 4 lines

=20
b) never expose key 0 to applications. Allocate key 2 as the default key and present the key numbers with an offset to userspace so key 2 appears as key 0 to user applications. =20

=20
this is complicated. offseting the number is not easy. if kernel has allocated key-2 and tells the user space to use key-0, userspace will use bit-0 and bit-1 of the AMR register to program the permissions on the key, and cpu will start applying those changes on key-0. Basically kernel, userspace and cpu will be out-of-sync. This is possible if the userspace uses a shift-aware library function to program the AMR. BTW: A library function integrated into glibc is yet to be defined AFAICK.

How is the application denied setting the permissions on key 0 if it can program the register directly?

Applications need the ability to associate an address-range with some key and latter revert to its initial default key. Pkey-0 comes close to providing this function but falls short, because the current implementation disallows applications to explicitly associate pkey-0 to the address range.

Lets make pkey-0 less special and treat it almost like any other key. Thus it can be explicitly associated with any address range, and can be freed. This gives the application more flexibility and power. The ability to free pkey-0 must be used responsibily, since pkey-0 is associated with almost all address-range by default.

Even with this change pkey-0 continues to be slightly more special from the following point of view.
(a) it is implicitly allocated.
(b) it is the default key assigned to any address-range.
(c) its permissions cannot be modified by userspace.

NOTE: (c) is specific to powerpc only. pkey-0 is associated by default with all pages including kernel pages, and pkeys are also active in kernel mode. If any permission is denied on pkey-0, the kernel running in the context of the application will be unable to operate.

Tested on powerpc.

cc Thomas Gleixner <[email protected]>

cc Dave Hansen <[email protected]>

cc Michael Ellermen <[email protected]>

cc Ingo Molnar <[email protected]>

cc Andrew Morton <[email protected]>

Signed-off-by Ram Pai <[email protected]>

---
History:

v3: . Corrected a comment in arch_set_user_pkey_access().
. Clarified the header, to capture the notion that
pkey-0 permissions cannot be modified by userspace on
powerpc. -- comment from Thiago

v2: . mm_pkey_is_allocated() continued to treat pkey-0
special. fixed it.

 arch/powerpc/include/asm/pkeys.h |   22 ++++++++++++++++++----
 arch/powerpc/mm/pkeys.c          |   26 +++++++++++++++-----------
 2 files changed, 33 insertions(+), 15 deletions(-)

(quoted diff) +0 -0

b/arch/powerpc/include/asm/pkeys.h index 0409c80..31a6976 100644

arch/powerpc/include/asm/pkeys.h +0 -1

@@ -101,10 +101,14 @@ 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' */

arch/powerpc/include/asm/pkeys.h +18 -3

-	return ((pkey < arch_max_pkey()) &&
-		!__mm_pkey_is_reserved(pkey) &&
-		__mm_pkey_is_allocated(mm, pkey));
+	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);

@@ -200,6 +204,16 @@ static inline int
 arch_set_user_pkey_access(struct task_struct *tsk, int pkey, {
 	if (static_branch_likely(&pkey_disabled))
 		return -EINVAL;
+
+	/*
+	 * userspace should not change pkey-0 permissions.
+	 * pkey-0 is associated with every page in the kernel.
+	 * If userspace denies any permission on pkey-0, the
+	 * kernel cannot operate.
+	 */
+	if (!pkey)
+		return init_val ? -EINVAL : 0;
+
 	return __arch_set_user_pkey_access(tsk, pkey, init_val);
 }
 

arch/powerpc/mm/pkeys.c +12 -7

index 0eafdf0..d6873b4 100644
@@ -119,16 +119,21 @@ int pkey_initialize(void)
 #else
 	os_reserved = 0;
 #endif
+	/* Bits are in LE format. */
 	initial_allocation_mask = ~0x0;
+
+	/* register mask is in BE format */
 	pkey_amr_uamor_mask = ~0x0ul;
 	pkey_iamr_mask = ~0x0ul;
-	/*
-	 * key 0, 1 are reserved.
-	 * key 0 is the default key, which allows read/write/execute.
-	 * key 1 is recommended not to be used. PowerISA(3.0) page
 1015,
-	 * programming note.
-	 */
-	for (i = 2; i < (pkeys_total - os_reserved); i++) {
+
+	for (i = 0; i < (pkeys_total - os_reserved); i++) {
+		/*
+		 * key 1 is recommended not to be used.
+		 * PowerISA(3.0) page 1015,
+		 */
+		if (i == 1)
+			continue;
+
 		initial_allocation_mask &= ~(0x1 << i);
 		pkey_amr_uamor_mask &= ~(0x3ul << pkeyshift(i));

arch/powerpc/mm/pkeys.c +3 -4

 		pkey_iamr_mask &= ~(0x1ul << pkeyshift(i));

@@ -142,7 +147,9 @@ void pkey_mm_init(struct mm_struct *mm)
 {
 	if (static_branch_likely(&pkey_disabled))
 		return;
-	mm_pkey_allocation_map(mm) = initial_allocation_mask;
+
+	/* allocate key-0 by default */
+	mm_pkey_allocation_map(mm) = initial_allocation_mask | 0x1;
 	/* -1 means unallocated or invalid */
 	mm->context.execute_only_pkey = -1;
 }

@@ -407,9 +414,6 @@ static bool pkey_access_permitted(int pkey, bool
 write, bool execute) int pkey_shift;
 	u64 amr;
 
-	if (!pkey)
-		return true;
-
 	if (!is_pkey_enabled(pkey))
 		return true;
 

It seems it is somehow assumed this is key 0 but I did not find it documented anywhere nor did I notice an interface for determining the default key.

Quoted 3 lines · On 05/09/2018 08:43 AM, Michal Such=C3=A1nek wrote:

It seems it is somehow assumed this is key 0 but I did not find it documented anywhere nor did I notice an interface for determining the default key. =20

=20
Does the manpage not count as documentation? :)
=20
"pkey 0 is used as the default key"
=20
https://manpages.debian.org/stretch/manpages/pkeys.7.en.html