x86/kprobes: Fix optprobe to detect INT3 padding correctly

Commit

7705dc855797 ("x86/vmlinux: Use INT3 instead of NOP for linker fill bytes")

changed the padding bytes between functions from NOP to INT3. However,
when optprobe decodes a target function it finds INT3 and gives up the
jump optimization.

Instead of giving up any INT3 detection, check whether the rest of the
bytes to the end of the function are INT3. If all of them are INT3,
those come from the linker. In that case, continue the optprobe jump
optimization.

[ bp: Massage commit message. ]

Fixes: 7705dc855797 ("x86/vmlinux: Use INT3 instead of NOP for linker fill bytes")
Reported-by: Adam Zabrocki <[e-mail]>
Signed-off-by: Masami Hiramatsu <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Reviewed-by: Steven Rostedt (VMware) <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Cc: [e-mail]
Link: https://lkml.kernel.org/r/160767025681.3880685.16021570341428835411.stgit@devnote2

arch/x86/kernel/kprobes/opt.c

···
 	return ret;
 }
 
+static bool is_padding_int3(unsigned long addr, unsigned long eaddr)
+{
+	unsigned char ops;
+
+	for (; addr < eaddr; addr++) {
+		if (get_kernel_nofault(ops, (void *)addr) < 0 ||
+		    ops != INT3_INSN_OPCODE)
+			return false;
+	}
+
+	return true;
+}
+
 /* Decode whole function to ensure any instructions don't jump into target */
 static int can_optimize(unsigned long paddr)
 {
···
 			return 0;
 		kernel_insn_init(&insn, (void *)recovered_insn, MAX_INSN_SIZE);
 		insn_get_length(&insn);
-		/* Another subsystem puts a breakpoint */
+		/*
+		 * In the case of detecting unknown breakpoint, this could be
+		 * a padding INT3 between functions. Let's check that all the
+		 * rest of the bytes are also INT3.
+		 */
 		if (insn.opcode.bytes[0] == INT3_INSN_OPCODE)
-			return 0;
+			return is_padding_int3(addr, paddr - offset + size) ? 1 : 0;
+
 		/* Recover address */
 		insn.kaddr = (void *)addr;
 		insn.next_byte = (void *)(addr + insn.length);