riscv: introduce interfaces to patch kernel code

On strict kernel memory permission, we couldn't patch code without
writable permission. Preserve two holes in fixmap area, so we can map
the kernel code temporarily to fixmap area, then patch the instructions.

We need two pages here because we support the compressed instruction, so
the instruction might be align to 2 bytes. When patching the 32-bit
length instruction which is 2 bytes alignment, it will across two pages.

Introduce two interfaces to patch kernel code:
riscv_patch_text_nosync:
 - patch code without synchronization, it's caller's responsibility to
   synchronize all CPUs if needed.
riscv_patch_text:
 - patch code and always synchronize with stop_machine()

Signed-off-by: Zong Li <zong.li@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/include/asm/fixmap.h

@@ -27,6 +27,8 @@ enum fixed_addresses {
 	FIX_FDT = FIX_FDT_END + FIX_FDT_SIZE / PAGE_SIZE - 1,
 	FIX_PTE,
 	FIX_PMD,
+	FIX_TEXT_POKE1,
+	FIX_TEXT_POKE0,
 	FIX_EARLYCON_MEM_BASE,
 	__end_of_fixed_addresses
 };

arch/riscv/include/asm/patch.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2020 SiFive
+ */
+
+#ifndef _ASM_RISCV_PATCH_H
+#define _ASM_RISCV_PATCH_H
+
+int riscv_patch_text_nosync(void *addr, const void *insns, size_t len);
+int riscv_patch_text(void *addr, u32 insn);
+
+#endif /* _ASM_RISCV_PATCH_H */

arch/riscv/kernel/Makefile

@@ -5,6 +5,7 @@
 
 ifdef CONFIG_FTRACE
 CFLAGS_REMOVE_ftrace.o	= -pg
+CFLAGS_REMOVE_patch.o	= -pg
 endif
 
 extra-y += head.o
@@ -26,6 +27,7 @@ obj-y	+= traps.o
 obj-y	+= riscv_ksyms.o
 obj-y	+= stacktrace.o
 obj-y	+= cacheinfo.o
+obj-y	+= patch.o
 obj-$(CONFIG_MMU) += vdso.o vdso/
 
 obj-$(CONFIG_RISCV_M_MODE)	+= clint.o

arch/riscv/kernel/patch.c

@@ -0,0 +1,120 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2020 SiFive
+ */
+
+#include <linux/spinlock.h>
+#include <linux/mm.h>
+#include <linux/uaccess.h>
+#include <linux/stop_machine.h>
+#include <asm/kprobes.h>
+#include <asm/cacheflush.h>
+#include <asm/fixmap.h>
+
+struct riscv_insn_patch {
+	void *addr;
+	u32 insn;
+	atomic_t cpu_count;
+};
+
+#ifdef CONFIG_MMU
+static DEFINE_RAW_SPINLOCK(patch_lock);
+
+static void __kprobes *patch_map(void *addr, int fixmap)
+{
+	uintptr_t uintaddr = (uintptr_t) addr;
+	struct page *page;
+
+	if (core_kernel_text(uintaddr))
+		page = phys_to_page(__pa_symbol(addr));
+	else if (IS_ENABLED(CONFIG_STRICT_MODULE_RWX))
+		page = vmalloc_to_page(addr);
+	else
+		return addr;
+
+	BUG_ON(!page);
+
+	return (void *)set_fixmap_offset(fixmap, page_to_phys(page) +
+					 (uintaddr & ~PAGE_MASK));
+}
+
+static void __kprobes patch_unmap(int fixmap)
+{
+	clear_fixmap(fixmap);
+}
+
+static int __kprobes riscv_insn_write(void *addr, const void *insn, size_t len)
+{
+	void *waddr = addr;
+	bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
+	unsigned long flags = 0;
+	int ret;
+
+	raw_spin_lock_irqsave(&patch_lock, flags);
+
+	if (across_pages)
+		patch_map(addr + len, FIX_TEXT_POKE1);
+
+	waddr = patch_map(addr, FIX_TEXT_POKE0);
+
+	ret = probe_kernel_write(waddr, insn, len);
+
+	patch_unmap(FIX_TEXT_POKE0);
+
+	if (across_pages)
+		patch_unmap(FIX_TEXT_POKE1);
+
+	raw_spin_unlock_irqrestore(&patch_lock, flags);
+
+	return ret;
+}
+#else
+static int __kprobes riscv_insn_write(void *addr, const void *insn, size_t len)
+{
+	return probe_kernel_write(addr, insn, len);
+}
+#endif /* CONFIG_MMU */
+
+int __kprobes riscv_patch_text_nosync(void *addr, const void *insns, size_t len)
+{
+	u32 *tp = addr;
+	int ret;
+
+	ret = riscv_insn_write(tp, insns, len);
+
+	if (!ret)
+		flush_icache_range((uintptr_t) tp, (uintptr_t) tp + len);
+
+	return ret;
+}
+
+static int __kprobes riscv_patch_text_cb(void *data)
+{
+	struct riscv_insn_patch *patch = data;
+	int ret = 0;
+
+	if (atomic_inc_return(&patch->cpu_count) == 1) {
+		ret =
+		    riscv_patch_text_nosync(patch->addr, &patch->insn,
+					    GET_INSN_LENGTH(patch->insn));
+		atomic_inc(&patch->cpu_count);
+	} else {
+		while (atomic_read(&patch->cpu_count) <= num_online_cpus())
+			cpu_relax();
+		smp_mb();
+	}
+
+	return ret;
+}
+
+int __kprobes riscv_patch_text(void *addr, u32 insn)
+{
+	struct riscv_insn_patch patch = {
+		.addr = addr,
+		.insn = insn,
+		.cpu_count = ATOMIC_INIT(0),
+	};
+
+	return stop_machine_cpuslocked(riscv_patch_text_cb,
+				       &patch, cpu_online_mask);
+}