[PATCH] powerpc/uprobes: Don't allow probe on suffix of prefixed instruction

[Ravi Bangoria <ravi.bangoria@linux.ibm.com>]

Probe on 2nd word of a prefixed instruction is invalid scenario and
should be restricted.

There are two ways probed instruction is changed in mapped pages.
First, when Uprobe is activated, it searches for all the relevant
pages and replace instruction in them. In this case, if we notice
that probe is on the 2nd word of prefixed instruction, error out
directly. Second, when Uprobe is already active and user maps a
relevant page via mmap(), instruction is replaced via mmap() code
path. But because Uprobe is invalid, entire mmap() operation can
not be stopped. In this case just print an error and continue.

Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
---
 arch/powerpc/kernel/uprobes.c | 28 ++++++++++++++++++++++++++++
 include/linux/uprobes.h       |  1 +
 kernel/events/uprobes.c       |  8 ++++++++
 3 files changed, 37 insertions(+)

diff --git a/arch/powerpc/kernel/uprobes.c b/arch/powerpc/kernel/uprobes.c
index e8a63713e655..c73d5a397164 100644
--- a/arch/powerpc/kernel/uprobes.c
+++ b/arch/powerpc/kernel/uprobes.c
@@ -7,6 +7,7 @@
  * Adapted from the x86 port by Ananth N Mavinakayanahalli <ananth@in.ibm.com>
  */
 #include <linux/kernel.h>
+#include <linux/highmem.h>
 #include <linux/sched.h>
 #include <linux/ptrace.h>
 #include <linux/uprobes.h>
@@ -44,6 +45,33 @@ int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe,
 	return 0;
 }
 
+#ifdef CONFIG_PPC64
+int arch_uprobe_verify_opcode(struct page *page, unsigned long vaddr,
+			      uprobe_opcode_t opcode)
+{
+	uprobe_opcode_t prefix;
+	void *kaddr;
+	struct ppc_inst inst;
+
+	/* Don't check if vaddr is pointing to the beginning of page */
+	if (!(vaddr & ~PAGE_MASK))
+		return 0;
+
+	kaddr = kmap_atomic(page);
+	memcpy(&prefix, kaddr + ((vaddr - 4) & ~PAGE_MASK), UPROBE_SWBP_INSN_SIZE);
+	kunmap_atomic(kaddr);
+
+	inst = ppc_inst_prefix(prefix, opcode);
+
+	if (ppc_inst_prefixed(inst)) {
+		printk_ratelimited("Cannot register a uprobe on the second "
+				   "word of prefixed instruction\n");
+		return -1;
+	}
+	return 0;
+}
+#endif
+
 /*
  * arch_uprobe_pre_xol - prepare to execute out of line.
  * @auprobe: the probepoint information.
diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h
index f46e0ca0169c..5a3b45878e13 100644
--- a/include/linux/uprobes.h
+++ b/include/linux/uprobes.h
@@ -128,6 +128,7 @@ extern bool uprobe_deny_signal(void);
 extern bool arch_uprobe_skip_sstep(struct arch_uprobe *aup, struct pt_regs *regs);
 extern void uprobe_clear_state(struct mm_struct *mm);
 extern int  arch_uprobe_analyze_insn(struct arch_uprobe *aup, struct mm_struct *mm, unsigned long addr);
+int arch_uprobe_verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t opcode);
 extern int  arch_uprobe_pre_xol(struct arch_uprobe *aup, struct pt_regs *regs);
 extern int  arch_uprobe_post_xol(struct arch_uprobe *aup, struct pt_regs *regs);
 extern bool arch_uprobe_xol_was_trapped(struct task_struct *tsk);
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index bf9edd8d75be..be02e6c26e3f 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -255,6 +255,12 @@ static void copy_to_page(struct page *page, unsigned long vaddr, const void *src
 	kunmap_atomic(kaddr);
 }
 
+int __weak arch_uprobe_verify_opcode(struct page *page, unsigned long vaddr,
+				     uprobe_opcode_t opcode)
+{
+	return 0;
+}
+
 static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t *new_opcode)
 {
 	uprobe_opcode_t old_opcode;
@@ -275,6 +281,8 @@ static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t
 	if (is_swbp_insn(new_opcode)) {
 		if (is_swbp)		/* register: already installed? */
 			return 0;
+		if (arch_uprobe_verify_opcode(page, vaddr, old_opcode))
+			return -EINVAL;
 	} else {
 		if (!is_swbp)		/* unregister: was it changed by us? */
 			return 0;
-- 
2.26.2