[PATCH v2] kprobes/x86: Use copy_from_kernel_nofault() to read from unsafe address

From: Masami Hiramatsu (Google) <mhiramat@kernel.org>

Read from an unsafe address with copy_from_kernel_nofault() in
arch_adjust_kprobe_addr() because this function is used before checking
the address is in text or not. Syzcaller bot found a bug and reported
the case if user specifies inaccessible data area,
arch_adjust_kprobe_addr() will cause a kernel panic.

Reported-by: Qiang Zhang <redacted>
Closes: https://lore.kernel.org/all/CAKHoSAs2rof6vQVBw_Lg_j3QNku0CANZR2qmy4eT7R5Lo8MFbg@mail.gmail.com/
Fixes: cc66bb914578 ("x86/ibt,kprobes: Cure sym+0 equals fentry woes")
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
---
 Changes in v2:
  - Fix to return NULL if failed to access the address.
---
 arch/x86/kernel/kprobes/core.c |   11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index a0ce46c0a2d8..95e4ebe5d514 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c

@@ -335,7 +335,16 @@ static int can_probe(unsigned long paddr)
 kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
 					 bool *on_func_entry)
 {
-	if (is_endbr(*(u32 *)addr)) {
+	u32 insn;
+
+	/*
+	 * Since addr is not guaranteed to be safely accessed yet, use
+	 * copy_from_kernel_nofault() to get the instruction.
+	 */
+	if (copy_from_kernel_nofault(&insn, (void *)addr, sizeof(u32)))
+		return NULL;
+
+	if (is_endbr(insn)) {
 		*on_func_entry = !offset || offset == 4;
 		if (*on_func_entry)
 			offset = 4;