powerpc/lib/code-patching: Use alternate map for patch_instruction()

This patch creates the window using text_poke_area, allocated via
get_vm_area(). text_poke_area is per CPU to avoid locking.
text_poke_area for each cpu is setup using late_initcall, prior to
setup of these alternate mapping areas, we continue to use direct
write to change/modify kernel text. With the ability to use alternate
mappings to write to kernel text, it provides us the freedom to then
turn text read-only and implement CONFIG_STRICT_KERNEL_RWX.

This code is CPU hotplug aware to ensure that the we have mappings for
any new cpus as they come online and tear down mappings for any CPUs
that go offline.

Signed-off-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/code-patching.c

@@ -12,23 +12,186 @@
 #include <linux/vmalloc.h>
 #include <linux/init.h>
 #include <linux/mm.h>
-#include <asm/page.h>
-#include <asm/code-patching.h>
+#include <linux/cpuhotplug.h>
+#include <linux/slab.h>
 #include <linux/uaccess.h>
 #include <linux/kprobes.h>
 
+#include <asm/pgtable.h>
+#include <asm/tlbflush.h>
+#include <asm/page.h>
+#include <asm/code-patching.h>
 
-int patch_instruction(unsigned int *addr, unsigned int instr)
+static int __patch_instruction(unsigned int *addr, unsigned int instr)
 {
 	int err;
 
 	__put_user_size(instr, addr, 4, err);
 	if (err)
 		return err;
-	asm ("dcbst 0, %0; sync; icbi 0,%0; sync; isync" : : "r" (addr));
+
+	asm ("dcbst 0, %0; sync; icbi 0,%0; sync; isync" :: "r" (addr));
+
 	return 0;
 }
 
+#ifdef CONFIG_STRICT_KERNEL_RWX
+static DEFINE_PER_CPU(struct vm_struct *, text_poke_area);
+
+static int text_area_cpu_up(unsigned int cpu)
+{
+	struct vm_struct *area;
+
+	area = get_vm_area(PAGE_SIZE, VM_ALLOC);
+	if (!area) {
+		WARN_ONCE(1, "Failed to create text area for cpu %d\n",
+			cpu);
+		return -1;
+	}
+	this_cpu_write(text_poke_area, area);
+
+	return 0;
+}
+
+static int text_area_cpu_down(unsigned int cpu)
+{
+	free_vm_area(this_cpu_read(text_poke_area));
+	return 0;
+}
+
+/*
+ * Run as a late init call. This allows all the boot time patching to be done
+ * simply by patching the code, and then we're called here prior to
+ * mark_rodata_ro(), which happens after all init calls are run. Although
+ * BUG_ON() is rude, in this case it should only happen if ENOMEM, and we judge
+ * it as being preferable to a kernel that will crash later when someone tries
+ * to use patch_instruction().
+ */
+static int __init setup_text_poke_area(void)
+{
+	BUG_ON(!cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
+		"powerpc/text_poke:online", text_area_cpu_up,
+		text_area_cpu_down));
+
+	return 0;
+}
+late_initcall(setup_text_poke_area);
+
+/*
+ * This can be called for kernel text or a module.
+ */
+static int map_patch_area(void *addr, unsigned long text_poke_addr)
+{
+	unsigned long pfn;
+	int err;
+
+	if (is_vmalloc_addr(addr))
+		pfn = vmalloc_to_pfn(addr);
+	else
+		pfn = __pa_symbol(addr) >> PAGE_SHIFT;
+
+	err = map_kernel_page(text_poke_addr, (pfn << PAGE_SHIFT),
+				pgprot_val(PAGE_KERNEL));
+
+	pr_devel("Mapped addr %lx with pfn %lx:%d\n", text_poke_addr, pfn, err);
+	if (err)
+		return -1;
+
+	return 0;
+}
+
+static inline int unmap_patch_area(unsigned long addr)
+{
+	pte_t *ptep;
+	pmd_t *pmdp;
+	pud_t *pudp;
+	pgd_t *pgdp;
+
+	pgdp = pgd_offset_k(addr);
+	if (unlikely(!pgdp))
+		return -EINVAL;
+
+	pudp = pud_offset(pgdp, addr);
+	if (unlikely(!pudp))
+		return -EINVAL;
+
+	pmdp = pmd_offset(pudp, addr);
+	if (unlikely(!pmdp))
+		return -EINVAL;
+
+	ptep = pte_offset_kernel(pmdp, addr);
+	if (unlikely(!ptep))
+		return -EINVAL;
+
+	pr_devel("clearing mm %p, pte %p, addr %lx\n", &init_mm, ptep, addr);
+
+	/*
+	 * In hash, pte_clear flushes the tlb, in radix, we have to
+	 */
+	pte_clear(&init_mm, addr, ptep);
+	flush_tlb_kernel_range(addr, addr + PAGE_SIZE);
+
+	return 0;
+}
+
+int patch_instruction(unsigned int *addr, unsigned int instr)
+{
+	int err;
+	unsigned int *dest = NULL;
+	unsigned long flags;
+	unsigned long text_poke_addr;
+	unsigned long kaddr = (unsigned long)addr;
+
+	/*
+	 * During early early boot patch_instruction is called
+	 * when text_poke_area is not ready, but we still need
+	 * to allow patching. We just do the plain old patching
+	 * We use slab_is_available and per cpu read * via this_cpu_read
+	 * of text_poke_area. Per-CPU areas might not be up early
+	 * this can create problems with just using this_cpu_read()
+	 */
+	if (!slab_is_available() || !this_cpu_read(text_poke_area))
+		return __patch_instruction(addr, instr);
+
+	local_irq_save(flags);
+
+	text_poke_addr = (unsigned long)__this_cpu_read(text_poke_area)->addr;
+	if (map_patch_area(addr, text_poke_addr)) {
+		err = -1;
+		goto out;
+	}
+
+	dest = (unsigned int *)(text_poke_addr) +
+			((kaddr & ~PAGE_MASK) / sizeof(unsigned int));
+
+	/*
+	 * We use __put_user_size so that we can handle faults while
+	 * writing to dest and return err to handle faults gracefully
+	 */
+	__put_user_size(instr, dest, 4, err);
+	if (!err)
+		asm ("dcbst 0, %0; sync; icbi 0,%0; icbi 0,%1; sync; isync"
+			::"r" (dest), "r"(addr));
+
+	err = unmap_patch_area(text_poke_addr);
+	if (err)
+		pr_warn("failed to unmap %lx\n", text_poke_addr);
+
+out:
+	local_irq_restore(flags);
+
+	return err;
+}
+#else /* !CONFIG_STRICT_KERNEL_RWX */
+
+int patch_instruction(unsigned int *addr, unsigned int instr)
+{
+	return __patch_instruction(addr, instr);
+}
+
+#endif /* CONFIG_STRICT_KERNEL_RWX */
+NOKPROBE_SYMBOL(patch_instruction);
+
 int patch_branch(unsigned int *addr, unsigned long target, int flags)
 {
 	return patch_instruction(addr, create_branch(addr, target, flags));