arm64: kprobe: make page to RO mode when allocate it

Commit 1404d6f1 ("arm64: dump: Add checking for writable and exectuable pages")
has successfully identified code that leaves a page with W+X
permissions.

[    3.245140] arm64/mm: Found insecure W+X mapping at address (____ptrval____)/0xffff000000d90000
[    3.245771] WARNING: CPU: 0 PID: 1 at ../arch/arm64/mm/dump.c:232 note_page+0x410/0x420
[    3.246141] Modules linked in:
[    3.246653] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.19.0-rc5-next-20180928-00001-ge70ae259b853-dirty #62
[    3.247008] Hardware name: linux,dummy-virt (DT)
[    3.247347] pstate: 80000005 (Nzcv daif -PAN -UAO)
[    3.247623] pc : note_page+0x410/0x420
[    3.247898] lr : note_page+0x410/0x420
[    3.248071] sp : ffff00000804bcd0
[    3.248254] x29: ffff00000804bcd0 x28: ffff000009274000
[    3.248578] x27: ffff00000921a000 x26: ffff80007dfff000
[    3.248845] x25: ffff0000093f5000 x24: ffff000009526f6a
[    3.249109] x23: 0000000000000004 x22: ffff000000d91000
[    3.249396] x21: ffff000000d90000 x20: 0000000000000000
[    3.249661] x19: ffff00000804bde8 x18: 0000000000000400
[    3.249924] x17: 0000000000000000 x16: 0000000000000000
[    3.250271] x15: ffffffffffffffff x14: 295f5f5f5f6c6176
[    3.250594] x13: 7274705f5f5f5f28 x12: 2073736572646461
[    3.250941] x11: 20746120676e6970 x10: 70616d20582b5720
[    3.251252] x9 : 6572756365736e69 x8 : 3039643030303030
[    3.251519] x7 : 306666666678302f x6 : ffff0000095467b2
[    3.251802] x5 : 0000000000000000 x4 : 0000000000000000
[    3.252060] x3 : 0000000000000000 x2 : ffffffffffffffff
[    3.252323] x1 : 4d151327adc50b00 x0 : 0000000000000000
[    3.252664] Call trace:
[    3.252953]  note_page+0x410/0x420
[    3.253186]  walk_pgd+0x12c/0x238
[    3.253417]  ptdump_check_wx+0x68/0xf8
[    3.253637]  mark_rodata_ro+0x68/0x98
[    3.253847]  kernel_init+0x38/0x160
[    3.254103]  ret_from_fork+0x10/0x18

kprobes allocates a writable executable page with module_alloc() in
order to store executable code.
Reworked to that when allocate a page it sets mode RO. Inspired by
commit 63fef14f ("kprobes/x86: Make insn buffer always ROX and use text_poke()").
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Suggested-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
[catalin.marinas@arm.com: removed unnecessary casts]
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/probes/kprobes.c

@@ -23,7 +23,9 @@
 #include <linux/slab.h>
 #include <linux/stop_machine.h>
 #include <linux/sched/debug.h>
+#include <linux/set_memory.h>
 #include <linux/stringify.h>
+#include <linux/vmalloc.h>
 #include <asm/traps.h>
 #include <asm/ptrace.h>
 #include <asm/cacheflush.h>
@@ -42,10 +44,21 @@ DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
 static void __kprobes
 post_kprobe_handler(struct kprobe_ctlblk *, struct pt_regs *);
 
+static int __kprobes patch_text(kprobe_opcode_t *addr, u32 opcode)
+{
+	void *addrs[1];
+	u32 insns[1];
+
+	addrs[0] = addr;
+	insns[0] = opcode;
+
+	return aarch64_insn_patch_text(addrs, insns, 1);
+}
+
 static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
 {
 	/* prepare insn slot */
-	p->ainsn.api.insn[0] = cpu_to_le32(p->opcode);
+	patch_text(p->ainsn.api.insn, p->opcode);
 
 	flush_icache_range((uintptr_t) (p->ainsn.api.insn),
 			   (uintptr_t) (p->ainsn.api.insn) +
@@ -118,15 +131,15 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
 	return 0;
 }
 
-static int __kprobes patch_text(kprobe_opcode_t *addr, u32 opcode)
+void *alloc_insn_page(void)
 {
-	void *addrs[1];
-	u32 insns[1];
+	void *page;
 
-	addrs[0] = (void *)addr;
-	insns[0] = (u32)opcode;
+	page = vmalloc_exec(PAGE_SIZE);
+	if (page)
+		set_memory_ro((unsigned long)page, 1);
 
-	return aarch64_insn_patch_text(addrs, insns, 1);
+	return page;
 }
 
 /* arm kprobe: install breakpoint in text */