"Debugging with GDB / Appendix E GDB Remote Serial Protocol /
Overview" specifies "The printable characters '#' and '$' or with a
numeric value greater than 126 must not be used."  gdb_read_byte()
only rejects values < 32.  This is wrong.  Impact depends on the caller:

* gdb_handlesig() passes a char.  Incorrectly accepts '#', '$' and
  '\127'.

* gdb_chr_receive() passes an uint8_t.  Additionally accepts
  characters with the most-significant bit set.

Correct the validity check to match the specification.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
---
 gdbstub.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/gdbstub.c b/gdbstub.c
index d54abd17cc..c41eb1de07 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -2064,7 +2064,11 @@ static void gdb_read_byte(GDBState *s, int ch)
             }
             break;
         case RS_GETLINE_RLE:
-            if (ch < ' ') {
+            /*
+             * Run-length encoding is explained in "Debugging with GDB /
+             * Appendix E GDB Remote Serial Protocol / Overview".
+             */
+            if (ch < ' ' || ch == '#' || ch == '$' || ch > 126) {
                 /* invalid RLE count encoding */
                 trace_gdbstub_err_invalid_repeat((uint8_t)ch);
                 s->state = RS_GETLINE;
-- 
2.17.2