[PATCH] uprop: Add new verification condition to verify_opcode

[Penglei Jiang <superman.xpt@gmail.com>]

If, during probe registration, the instruction at the probe point
differs between the file and memory (due to self-modifying code),
and the probe registration succeeds, the following errors occur:

  1. When unregistering the probe, it restores the wrong original
     instruction.
     - If the instruction crosses a page boundary: it restores only
       part of the instruction fetched from the file (the part saved
       in the first page) to the probe point, and does not restore
       the remaining part that crosses the page boundary, which can
       lead to unknown instruction.
     - If the instruction does not cross a page boundary: it restores
       the instruction fetched from the file to the probe point.

  2. The code logic changes because the saved original instruction
     is fetched from the file.

Fix:

  Add an extra check in the registration branch of verify_opcode.
  If the instruction at the probe point differs between the file
  and memory, the verification fails.

Signed-off-by: Penglei Jiang <superman.xpt@gmail.com>
---
 kernel/events/uprobes.c | 47 +++++++++++++++++++++++++++++++++++++++--
 1 file changed, 45 insertions(+), 2 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index b4ca8898fe17..580dd9fe4015 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -264,10 +264,46 @@ static void copy_to_page(struct page *page, unsigned long vaddr, const void *src
 	kunmap_atomic(kaddr);
 }
 
-static int verify_opcode(struct page *page, unsigned long vaddr, uprobe_opcode_t *new_opcode)
+static int copy_insn_from_mem(struct page *p, unsigned long vaddr,
+		struct arch_uprobe *dst, struct mm_struct *mm)
+{
+	pgoff_t offset = offset_in_page(vaddr);
+	void *dst_insn = &dst->insn;
+	size_t size = sizeof(dst->insn);
+	size_t first_size;
+	int err;
+
+	first_size = min_t(size_t, size, PAGE_SIZE - offset);
+	copy_from_page(p, offset, dst_insn, first_size);
+	vaddr += first_size;
+	dst_insn += first_size;
+	size -= first_size;
+
+	/*
+	 * If the instruction spans the page boundary, continue copying
+	 * the remaining instruction from the second page.
+	 */
+	if (size) {
+		err = get_user_pages_remote(mm, vaddr, 1, FOLL_FORCE, &p, NULL);
+		if (err < 0)
+			return err;
+		if (err == 0)
+			return -EBUSY;
+
+		copy_from_page(p, 0, dst_insn, size);
+		put_page(p);
+	}
+
+	return 0;
+}
+
+static int verify_opcode(struct page *page, unsigned long vaddr,
+		uprobe_opcode_t *new_opcode, const struct arch_uprobe *auprobe,
+		struct mm_struct *mm)
 {
 	uprobe_opcode_t old_opcode;
 	bool is_swbp;
+	struct arch_uprobe current_insn;
 
 	/*
 	 * Note: We only check if the old_opcode is UPROBE_SWBP_INSN here.
@@ -284,6 +320,13 @@ 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 (copy_insn_from_mem(page, vaddr, &current_insn, mm))
+			return 0;
+					/* register: was it changed by self-modifying code? */
+		if (memcmp(&current_insn.insn, &auprobe->insn,
+			sizeof(current_insn.insn)))
+			return 0;
 	} else {
 		if (!is_swbp)		/* unregister: was it changed by us? */
 			return 0;
@@ -491,7 +534,7 @@ int uprobe_write_opcode(struct arch_uprobe *auprobe, struct mm_struct *mm,
 	if (IS_ERR(old_page))
 		return PTR_ERR(old_page);
 
-	ret = verify_opcode(old_page, vaddr, &opcode);
+	ret = verify_opcode(old_page, vaddr, &opcode, auprobe, mm);
 	if (ret <= 0)
 		goto put_old;
 
-- 
2.17.1