arm: use fixmap for text patching when text is RO

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>
[kees: fixed up for merge with "arm: use generic fixmap.h"]
[kees: added parse acquire/release annotations to pass C=1 builds]
[kees: always use stop_machine to keep TLB flushing local]
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Nicolas Pitre <nico@linaro.org>

4 files changed, 89 insertions, 21 deletions

diff --git a/arch/arm/include/asm/fixmap.h b/arch/arm/include/asm/fixmap.h
index 714606f70425..0415eae1df27 100644
--- a/arch/arm/include/asm/fixmap.h
+++ b/arch/arm/include/asm/fixmap.h
@@ -11,6 +11,10 @@ enum fixed_addresses {
         FIX_KMAP_BEGIN,
         FIX_KMAP_END = FIX_KMAP_BEGIN + (KM_TYPE_NR * NR_CPUS) - 1,
 
+        /* Support writing RO kernel text via kprobes, jump labels, etc. */
+        FIX_TEXT_POKE0,
+        FIX_TEXT_POKE1,
+
         __end_of_fixed_addresses
 };
 
diff --git a/arch/arm/kernel/jump_label.c b/arch/arm/kernel/jump_label.c
index 4ce4f789446d..afeeb9ea6f43 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 07314af47733..5038960e3c55 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 +16,77 @@ 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)
+        __acquires(&patch_lock)
+{
+        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_DEBUG_RODATA))
+                page = virt_to_page(addr);
+        else
+                return addr;
+
+        if (flags)
+                spin_lock_irqsave(&patch_lock, *flags);
+        else
+                __acquire(&patch_lock);
+
+        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)
+        __releases(&patch_lock)
+{
+        clear_fixmap(fixmap);
+
+        if (flags)
+                spin_unlock_irqrestore(&patch_lock, *flags);
+        else
+                __release(&patch_lock);
+}
+
+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;
+        bool twopage = false;
+        unsigned long flags;
+        void *waddr = addr;
         int size;
 
+        if (remap)
+                waddr = patch_map(addr, FIX_TEXT_POKE0, &flags);
+        else
+                __acquire(&patch_lock);
+
         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)) {
                 u16 first = __opcode_thumb32_first(insn);
                 u16 second = __opcode_thumb32_second(insn);
-                u16 *addrh = addr;
+                u16 *addrh0 = waddr;
+                u16 *addrh1 = waddr + 2;
+
+                twopage = (uintaddr & ~PAGE_MASK) == PAGE_SIZE - 2;
+                if (twopage && remap)
+                        addrh1 = patch_map(addr + 2, FIX_TEXT_POKE1, NULL);
+
+                *addrh0 = __opcode_to_mem_thumb16(first);
+                *addrh1 = __opcode_to_mem_thumb16(second);
 
-                addrh[0] = __opcode_to_mem_thumb16(first);
-                addrh[1] = __opcode_to_mem_thumb16(second);
+                if (twopage && addrh1 != addr + 2) {
+                        flush_kernel_vmap_range(addrh1, 2);
+                        patch_unmap(FIX_TEXT_POKE1, NULL);
+                }
 
                 size = sizeof(u32);
         } else {
@@ -36,10 +95,16 @@ 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) {
+                flush_kernel_vmap_range(waddr, twopage ? size / 2 : size);
+                patch_unmap(FIX_TEXT_POKE0, &flags);
+        } else
+                __release(&patch_lock);
+
         flush_icache_range((uintptr_t)(addr),
                            (uintptr_t)(addr) + size);
 }
@@ -60,16 +125,5 @@ void __kprobes patch_text(void *addr, unsigned int insn)
                 .insn = insn,
         };
 
-        if (cache_ops_need_broadcast()) {
-                stop_machine(patch_text_stop_machine, &patch, cpu_online_mask);
-        } else {
-                bool straddles_word = IS_ENABLED(CONFIG_THUMB2_KERNEL)
-                                      && __opcode_is_thumb32(insn)
-                                      && ((uintptr_t)addr & 2);
-
-                if (straddles_word)
-                        stop_machine(patch_text_stop_machine, &patch, NULL);
-                else
-                        __patch_text(addr, insn);
-        }
+        stop_machine(patch_text_stop_machine, &patch, NULL);
 }
diff --git a/arch/arm/kernel/patch.h b/arch/arm/kernel/patch.h
index b4731f2dac38..77e054c2f6cd 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