powerpc: introduce execute-only pkey

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.

Acked-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/book3s/64/mmu.h

@@ -116,6 +116,7 @@ typedef struct {
 	 * bit unset -> key available for allocation
 	 */
 	u32 pkey_allocation_map;
+	s16 execute_only_pkey; /* key holding execute-only protection */
 #endif
 } mm_context_t;
 

arch/powerpc/include/asm/pkeys.h

@@ -117,9 +117,13 @@ 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 (static_branch_likely(&pkey_disabled))
+		return -1;
+
+	return __execute_only_pkey(mm);
 }
 
 static inline int arch_override_mprotect_pkey(struct vm_area_struct *vma,

arch/powerpc/mm/pkeys.c

@@ -92,6 +92,8 @@ void pkey_mm_init(struct mm_struct *mm)
 	if (static_branch_likely(&pkey_disabled))
 		return;
 	mm_pkey_allocation_map(mm) = initial_allocation_mask;
+	/* -1 means unallocated or invalid */
+	mm->context.execute_only_pkey = -1;
 }
 
 static inline u64 read_amr(void)
@@ -256,3 +258,59 @@ void thread_pkey_regs_init(struct thread_struct *thread)
 	write_iamr(read_iamr() & pkey_iamr_mask);
 	write_uamor(read_uamor() & pkey_amr_uamor_mask);
 }
+
+static inline bool pkey_allows_readwrite(int pkey)
+{
+	int pkey_shift = pkeyshift(pkey);
+
+	if (!is_pkey_enabled(pkey))
+		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_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;
+}