powerpc/mm: implement set_memory_attr()

In addition to the set_memory_xx() functions which allows to change
the memory attributes of not (yet) used memory regions, implement a
set_memory_attr() function to:
- set the final memory protection after init on currently used
kernel regions.
- enable/disable kernel memory regions in the scope of DEBUG_PAGEALLOC.

Unlike the set_memory_xx() which can act in three step as the regions
are unused, this function must modify 'on the fly' as the kernel is
executing from them. At the moment only PPC32 will use it and changing
page attributes on the fly is not an issue.

Reported-by: kbuild test robot <lkp@intel.com>
[ruscur: cast "data" to unsigned long instead of int]
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210609013431.9805-9-jniethe5@gmail.com

arch/powerpc/include/asm/set_memory.h

@@ -29,4 +29,6 @@ static inline int set_memory_x(unsigned long addr, int numpages)
 	return change_memory_attr(addr, numpages, SET_MEMORY_X);
 }
 
+int set_memory_attr(unsigned long addr, int numpages, pgprot_t prot);
+
 #endif

arch/powerpc/mm/pageattr.c

@@ -99,3 +99,36 @@ int change_memory_attr(unsigned long addr, int numpages, long action)
 	return apply_to_existing_page_range(&init_mm, start, size,
 					    change_page_attr, (void *)action);
 }
+
+/*
+ * Set the attributes of a page:
+ *
+ * This function is used by PPC32 at the end of init to set final kernel memory
+ * protection. It includes changing the maping of the page it is executing from
+ * and data pages it is using.
+ */
+static int set_page_attr(pte_t *ptep, unsigned long addr, void *data)
+{
+	pgprot_t prot = __pgprot((unsigned long)data);
+
+	spin_lock(&init_mm.page_table_lock);
+
+	set_pte_at(&init_mm, addr, ptep, pte_modify(*ptep, prot));
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+	spin_unlock(&init_mm.page_table_lock);
+
+	return 0;
+}
+
+int set_memory_attr(unsigned long addr, int numpages, pgprot_t prot)
+{
+	unsigned long start = ALIGN_DOWN(addr, PAGE_SIZE);
+	unsigned long sz = numpages * PAGE_SIZE;
+
+	if (numpages <= 0)
+		return 0;
+
+	return apply_to_existing_page_range(&init_mm, start, sz, set_page_attr,
+					    (void *)pgprot_val(prot));
+}