x86/kprobes: Fix optprobe to detect INT3 padding correctly

commit 0d07c0ec4381f630c801539c79ad8dcc627f6e4a upstream.

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 <pi3@pi3.com.pl>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/160767025681.3880685.16021570341428835411.stgit@devnote2
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 40f380461e6d7486fd81b6a803ce898f363f3ef6..bfb59a3f0085d33710161490fc4a133686bd32d0 100644 (file)
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -271,6 +271,19 @@ static int insn_is_indirect_jump(struct insn *insn)
        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)
 {
@@ -309,9 +322,14 @@ 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);