powerpc/mm: Mark __init memory no-execute when STRICT_KERNEL_RWX=y

Currently even with STRICT_KERNEL_RWX we leave the __init text marked
executable after init, which is bad.

Add a hook to mark it NX (no-execute) before we free it, and implement
it for radix and hash.

Note that we use __init_end as the end address, not _einittext,
because overlaps_kernel_text() uses __init_end, because there are
additional executable sections other than .init.text between
__init_begin and __init_end.

Tested on radix and hash with:

  0:mon> p $__init_begin
  *** 400 exception occurred

Fixes: 1e0fc9d1eb ("powerpc/Kconfig: Enable STRICT_KERNEL_RWX for some configs")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

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

@@ -91,6 +91,7 @@ static inline int hash__pgd_bad(pgd_t pgd)
 }
 #ifdef CONFIG_STRICT_KERNEL_RWX
 extern void hash__mark_rodata_ro(void);
+extern void hash__mark_initmem_nx(void);
 #endif
 
 extern void hpte_need_flush(struct mm_struct *mm, unsigned long addr,

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

@@ -1192,5 +1192,6 @@ static inline const int pud_pfn(pud_t pud)
 	BUILD_BUG();
 	return 0;
 }
+
 #endif /* __ASSEMBLY__ */
 #endif /* _ASM_POWERPC_BOOK3S_64_PGTABLE_H_ */

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

@@ -118,6 +118,7 @@
 
 #ifdef CONFIG_STRICT_KERNEL_RWX
 extern void radix__mark_rodata_ro(void);
+extern void radix__mark_initmem_nx(void);
 #endif
 
 static inline unsigned long __radix_pte_update(pte_t *ptep, unsigned long clr,

arch/powerpc/include/asm/pgtable.h

@@ -80,6 +80,13 @@ unsigned long vmalloc_to_phys(void *vmalloc_addr);
 
 void pgtable_cache_add(unsigned shift, void (*ctor)(void *));
 void pgtable_cache_init(void);
+
+#ifdef CONFIG_STRICT_KERNEL_RWX
+void mark_initmem_nx(void);
+#else
+static inline void mark_initmem_nx(void) { }
+#endif
+
 #endif /* __ASSEMBLY__ */
 
 #endif /* _ASM_POWERPC_PGTABLE_H */

arch/powerpc/mm/mem.c

@@ -402,6 +402,7 @@ void __init mem_init(void)
 void free_initmem(void)
 {
 	ppc_md.progress = ppc_printk_progress;
+	mark_initmem_nx();
 	free_initmem_default(POISON_FREE_INITMEM);
 }
 

arch/powerpc/mm/pgtable-hash64.c

@@ -460,4 +460,16 @@ void hash__mark_rodata_ro(void)
 
 	WARN_ON(!hash__change_memory_range(start, end, PP_RXXX));
 }
+
+void hash__mark_initmem_nx(void)
+{
+	unsigned long start, end, pp;
+
+	start = (unsigned long)__init_begin;
+	end = (unsigned long)__init_end;
+
+	pp = htab_convert_pte_flags(pgprot_val(PAGE_KERNEL));
+
+	WARN_ON(!hash__change_memory_range(start, end, pp));
+}
 #endif

arch/powerpc/mm/pgtable-radix.c

@@ -162,6 +162,14 @@ void radix__mark_rodata_ro(void)
 
 	radix__change_memory_range(start, end, _PAGE_WRITE);
 }
+
+void radix__mark_initmem_nx(void)
+{
+	unsigned long start = (unsigned long)__init_begin;
+	unsigned long end = (unsigned long)__init_end;
+
+	radix__change_memory_range(start, end, _PAGE_EXEC);
+}
 #endif /* CONFIG_STRICT_KERNEL_RWX */
 
 static inline void __meminit print_mapping(unsigned long start,

arch/powerpc/mm/pgtable_64.c

@@ -505,4 +505,12 @@ void mark_rodata_ro(void)
 	else
 		hash__mark_rodata_ro();
 }
+
+void mark_initmem_nx(void)
+{
+	if (radix_enabled())
+		radix__mark_initmem_nx();
+	else
+		hash__mark_initmem_nx();
+}
 #endif