arm64: Fix text patching logic when using fixmap

Patch 2f896d58 ("arm64: use fixmap for text patching") changed
the way we patch the kernel text, using a fixmap when the kernel or
modules are flagged as read only.

Unfortunately, a flaw in the logic makes it fall over when patching
modules without CONFIG_DEBUG_SET_MODULE_RONX enabled:

[...]
[   32.032636] Call trace:
[   32.032716] [<fffffe00003da0dc>] __copy_to_user+0x2c/0x60
[   32.032837] [<fffffe0000099f08>] __aarch64_insn_write+0x94/0xf8
[   32.033027] [<fffffe000009a0a0>] aarch64_insn_patch_text_nosync+0x18/0x58
[   32.033200] [<fffffe000009c3ec>] ftrace_modify_code+0x58/0x84
[   32.033363] [<fffffe000009c4e4>] ftrace_make_nop+0x3c/0x58
[   32.033532] [<fffffe0000164420>] ftrace_process_locs+0x3d0/0x5c8
[   32.033709] [<fffffe00001661cc>] ftrace_module_init+0x28/0x34
[   32.033882] [<fffffe0000135148>] load_module+0xbb8/0xfc4
[   32.034044] [<fffffe0000135714>] SyS_finit_module+0x94/0xc4
[...]

This is triggered by the use of virt_to_page() on a module address,
which ends to pointing to Nowhereland if you're lucky, or corrupt
your precious data if not.

This patch fixes the logic by mimicking what is done on arm:
- If we're patching a module and CONFIG_DEBUG_SET_MODULE_RONX is set,
  use vmalloc_to_page().
- If we're patching the kernel and CONFIG_DEBUG_RODATA is set,
  use virt_to_page().
- Otherwise, use the provided address, as we can write to it directly.

Tested on 4.0-rc1 as a KVM guest.
Reported-by: Richard W.M. Jones <rjones@redhat.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Laura Abbott <lauraa@codeaurora.org>
Tested-by: Richard W.M. Jones <rjones@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/insn.c

@@ -87,8 +87,10 @@ static void __kprobes *patch_map(void *addr, int fixmap)
 
 	if (module && IS_ENABLED(CONFIG_DEBUG_SET_MODULE_RONX))
 		page = vmalloc_to_page(addr);
-	else
+	else if (!module && IS_ENABLED(CONFIG_DEBUG_RODATA))
 		page = virt_to_page(addr);
+	else
+		return addr;
 
 	BUG_ON(!page);
 	set_fixmap(fixmap, page_to_phys(page));