[PATCH v3] powerpc, pkey: make protection key 0 less special

[Ram Pai <linuxram@us.ibm.com>]

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 <tglx@linutronix.de>
cc: Dave Hansen <dave.hansen@intel.com>
cc: Michael Ellermen <mpe@ellerman.id.au>
cc: Ingo Molnar <mingo@kernel.org>
cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
---
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(-)

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 0409c80..31a6976 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h
@@ -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' */
-	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);
 }
 
diff --git a/arch/powerpc/mm/pkeys.c b/arch/powerpc/mm/pkeys.c
index 0eafdf0..d6873b4 100644
--- a/arch/powerpc/mm/pkeys.c
+++ b/arch/powerpc/mm/pkeys.c
@@ -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));
 		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;
 
-- 
1.7.1