[PATCH v9 3/8] powerpc/kprobes: Mark newly allocated probes as RO

[Jordan Niethe <jniethe5@gmail.com>]

From: Russell Currey <ruscur@russell.cc>

With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
W+X page at boot by default.  This can be tested with
CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
kernel log during boot.

Add an arch specific insn page allocator which returns RO pages if
STRICT_KERNEL_RWX is enabled. This page is only written to with
patch_instruction() which is able to write RO pages.

Reviewed-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Russell Currey <ruscur@russell.cc>
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
[jpn: Reword commit message, switch from vmalloc_exec(), add
      free_insn_page()]
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
---
v9: - vmalloc_exec() no longer exists
    - Set the page to RW before freeing it
---
 arch/powerpc/kernel/kprobes.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index 01ab2163659e..bb7e4d321988 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c
@@ -25,6 +25,8 @@
 #include <asm/sections.h>
 #include <asm/inst.h>
 #include <linux/uaccess.h>
+#include <linux/set_memory.h>
+#include <linux/vmalloc.h>
 
 DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
 DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
@@ -103,6 +105,26 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
 	return addr;
 }
 
+void *alloc_insn_page(void)
+{
+	void *page = vmalloc(PAGE_SIZE);
+
+	if (!page)
+		return NULL;
+
+	set_memory_ro((unsigned long)page, 1);
+	set_memory_x((unsigned long)page, 1);
+
+	return page;
+}
+
+void free_insn_page(void *page)
+{
+	set_memory_nx((unsigned long)page, 1);
+	set_memory_rw((unsigned long)page, 1);
+	vfree(page);
+}
+
 int arch_prepare_kprobe(struct kprobe *p)
 {
 	int ret = 0;
-- 
2.25.1