[PATCH 2/2] arm: use fixmap for text patching when text is RO

[Rabin Vincent <rabin@rab.in>]

Use fixmaps for text patching when the kernel text is read-only,
inspired by x86.  This makes jump labels and kprobes work with the
currently available CONFIG_DEBUG_SET_MODULE_RONX and the upcoming
CONFIG_DEBUG_RODATA options.

Signed-off-by: Rabin Vincent <rabin@rab.in>
---
 arch/arm/include/asm/fixmap.h |  3 ++
 arch/arm/kernel/jump_label.c  |  2 +-
 arch/arm/kernel/patch.c       | 66 ++++++++++++++++++++++++++++++++++++++-----
 arch/arm/kernel/patch.h       | 12 +++++++-
 4 files changed, 74 insertions(+), 9 deletions(-)

diff --git a/arch/arm/include/asm/fixmap.h b/arch/arm/include/asm/fixmap.h
index 55ed076..79c1719 100644
--- a/arch/arm/include/asm/fixmap.h
+++ b/arch/arm/include/asm/fixmap.h
@@ -18,6 +18,9 @@
 #define FIXADDR_TOP		(FIXADDR_END - PAGE_SIZE)
 
 enum fixed_addresses {
+	FIX_TEXT_POKE0,
+	FIX_TEXT_POKE1,
+
 	FIX_KMAP_BEGIN,
 	FIX_KMAP_END = (FIXADDR_TOP - FIXADDR_START) >> PAGE_SHIFT,
 	__end_of_fixed_addresses
diff --git a/arch/arm/kernel/jump_label.c b/arch/arm/kernel/jump_label.c
index 4ce4f78..afeeb9e 100644
--- a/arch/arm/kernel/jump_label.c
+++ b/arch/arm/kernel/jump_label.c
@@ -19,7 +19,7 @@ static void __arch_jump_label_transform(struct jump_entry *entry,
 		insn = arm_gen_nop();
 
 	if (is_static)
-		__patch_text(addr, insn);
+		__patch_text_early(addr, insn);
 	else
 		patch_text(addr, insn);
 }
diff --git a/arch/arm/kernel/patch.c b/arch/arm/kernel/patch.c
index 07314af..761c5b6 100644
--- a/arch/arm/kernel/patch.c
+++ b/arch/arm/kernel/patch.c
@@ -1,8 +1,11 @@
 #include <linux/kernel.h>
+#include <linux/spinlock.h>
 #include <linux/kprobes.h>
+#include <linux/mm.h>
 #include <linux/stop_machine.h>
 
 #include <asm/cacheflush.h>
+#include <asm/fixmap.h>
 #include <asm/smp_plat.h>
 #include <asm/opcodes.h>
 
@@ -13,21 +16,67 @@ struct patch {
 	unsigned int insn;
 };
 
-void __kprobes __patch_text(void *addr, unsigned int insn)
+static DEFINE_SPINLOCK(patch_lock);
+
+static void __kprobes *patch_map(void *addr, int fixmap, unsigned long *flags)
+{
+	unsigned int uintaddr = (uintptr_t) addr;
+	bool module = !core_kernel_text(uintaddr);
+	struct page *page;
+
+	if (module && IS_ENABLED(CONFIG_DEBUG_SET_MODULE_RONX)) {
+		page = vmalloc_to_page(addr);
+	} else if (!module && IS_ENABLED(CONFIG_ARM_KERNMEM_PERMS)) {
+		page = virt_to_page(addr);
+	} else {
+		return addr;
+	}
+
+	if (flags)
+		spin_lock_irqsave(&patch_lock, *flags);
+
+	set_fixmap(fixmap, page_to_phys(page));
+
+	return (void *) (__fix_to_virt(fixmap) + (uintaddr & ~PAGE_MASK));
+}
+
+static void __kprobes patch_unmap(int fixmap, unsigned long *flags)
+{
+	clear_fixmap(fixmap);
+
+	if (flags)
+		spin_unlock_irqrestore(&patch_lock, *flags);
+}
+
+void __kprobes __patch_text_real(void *addr, unsigned int insn, bool remap)
 {
 	bool thumb2 = IS_ENABLED(CONFIG_THUMB2_KERNEL);
+	unsigned int uintaddr = (uintptr_t) addr;
+	unsigned long flags;
+	void *waddr = addr;
 	int size;
 
+	if (remap)
+		waddr = patch_map(addr, FIX_TEXT_POKE0, &flags);
+
 	if (thumb2 && __opcode_is_thumb16(insn)) {
-		*(u16 *)addr = __opcode_to_mem_thumb16(insn);
+		*(u16 *)waddr = __opcode_to_mem_thumb16(insn);
 		size = sizeof(u16);
-	} else if (thumb2 && ((uintptr_t)addr & 2)) {
+	} else if (thumb2 && (uintaddr & 2)) {
+		bool twopage = (uintaddr & ~PAGE_MASK) == PAGE_SIZE - 2;
 		u16 first = __opcode_thumb32_first(insn);
 		u16 second = __opcode_thumb32_second(insn);
-		u16 *addrh = addr;
+		u16 *addrh0 = waddr;
+		u16 *addrh1 = waddr + 2;
+
+		if (remap && twopage)
+			addrh1 = patch_map(addr + 2, FIX_TEXT_POKE1, NULL);
 
-		addrh[0] = __opcode_to_mem_thumb16(first);
-		addrh[1] = __opcode_to_mem_thumb16(second);
+		*addrh0 = __opcode_to_mem_thumb16(first);
+		*addrh1 = __opcode_to_mem_thumb16(second);
+
+		if (twopage && addrh1 != addr + 2)
+			patch_unmap(FIX_TEXT_POKE1, NULL);
 
 		size = sizeof(u32);
 	} else {
@@ -36,10 +85,13 @@ void __kprobes __patch_text(void *addr, unsigned int insn)
 		else
 			insn = __opcode_to_mem_arm(insn);
 
-		*(u32 *)addr = insn;
+		*(u32 *)waddr = insn;
 		size = sizeof(u32);
 	}
 
+	if (waddr != addr)
+		patch_unmap(FIX_TEXT_POKE0, &flags);
+
 	flush_icache_range((uintptr_t)(addr),
 			   (uintptr_t)(addr) + size);
 }
diff --git a/arch/arm/kernel/patch.h b/arch/arm/kernel/patch.h
index b4731f2..77e054c 100644
--- a/arch/arm/kernel/patch.h
+++ b/arch/arm/kernel/patch.h
@@ -2,6 +2,16 @@
 #define _ARM_KERNEL_PATCH_H
 
 void patch_text(void *addr, unsigned int insn);
-void __patch_text(void *addr, unsigned int insn);
+void __patch_text_real(void *addr, unsigned int insn, bool remap);
+
+static inline void __patch_text(void *addr, unsigned int insn)
+{
+	__patch_text_real(addr, insn, true);
+}
+
+static inline void __patch_text_early(void *addr, unsigned int insn)
+{
+	__patch_text_real(addr, insn, false);
+}
 
 #endif
-- 
1.9.1