[Qemu-devel] [PATCH] Limit memory r/w length to buffer size

[P J P <ppandit@redhat.com>]

    Hello,

An OOB r/w access issue was reported by Mr Gerben Lubbe(CC'd here).

The GDB(1) stub protocol supports commands 'm/M' to read & write 'len' bytes 
from/to the stub memory area. In that, the 'len' parameter value supplied by 
the host gdb(1) is not validated against the local buffer size. Which in turn 
could lead to OOB r/w memory access.

Below is a proposed patch to fix this issue.

===
>From 88edb457a66f8ff96209a1603914171eade0658b Mon Sep 17 00:00:00 2001
From: Prasad J Pandit <pjp@fedoraproject.org>
Date: Mon, 12 Oct 2015 22:56:41 +0530
Subject: Limit memory r/w length to buffer size

GDB(1) stub communication protocol supports commands m/M to read
and write 'len' bytes from/to the stub memory area.

    m addr,len        : read 'len' bytes from address 'addr'
    M addr,len:<data> : write 'len' bytes of 'data' to 'addr'

Qemu stub uses automatic buffers of size 'MAX_PACKET_LENGTH=4096'
to process these commands. Limit 'len' parameter value supplied
by the host gdb(1) to the maximum buffer size to avoid any OOB
buffer access.

Reported-by: Gerben van der Lubbe <spoofedexistence@gmail.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
  gdbstub.c | 2 ++
  1 file changed, 2 insertions(+)

diff --git a/gdbstub.c b/gdbstub.c
index ffe7e6e..39da736 100644
--- a/gdbstub.c
+++ b/gdbstub.c
@@ -956,6 +956,7 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
          if (*p == ',')
              p++;
          len = strtoull(p, NULL, 16);
+        len = len > MAX_PACKET_LENGTH ? MAX_PACKET_LENGTH : len;
          if (target_memory_rw_debug(s->g_cpu, addr, mem_buf, len, false) != 0) {
              put_packet (s, "E14");
          } else {
@@ -968,6 +969,7 @@ static int gdb_handle_packet(GDBState *s, const char *line_buf)
          if (*p == ',')
              p++;
          len = strtoull(p, (char **)&p, 16);
+        len = len > MAX_PACKET_LENGTH ? MAX_PACKET_LENGTH : len;
          if (*p == ':')
              p++;
          hextomem(mem_buf, p, len);
-- 
2.4.3
===


Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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