[PULL 3/6] 9pfs: validate count sent by client with T_readdir

[Greg Kurz <groug@kaod.org>]

From: Christian Schoenebeck <qemu_oss@crudebyte.com>

A good 9p client sends T_readdir with "count" parameter that's sufficiently
smaller than client's initially negotiated msize (maximum message size).
We perform a check for that though to avoid the server to be interrupted
with a "Failed to encode VirtFS reply type 41" transport error message by
bad clients. This count value constraint uses msize - 11, because 11 is the
header size of R_readdir.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Message-Id: <3990d3891e8ae2074709b56449e96ab4b4b93b7d.1579567020.git.qemu_oss@crudebyte.com>
[groug: added comment ]
Signed-off-by: Greg Kurz <groug@kaod.org>
---
 hw/9pfs/9p.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index c63f549f39b2..9e046f7acb51 100644
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -2434,6 +2434,7 @@ static void coroutine_fn v9fs_readdir(void *opaque)
     int32_t count;
     uint32_t max_count;
     V9fsPDU *pdu = opaque;
+    V9fsState *s = pdu->s;
 
     retval = pdu_unmarshal(pdu, offset, "dqd", &fid,
                            &initial_offset, &max_count);
@@ -2442,6 +2443,14 @@ static void coroutine_fn v9fs_readdir(void *opaque)
     }
     trace_v9fs_readdir(pdu->tag, pdu->id, fid, initial_offset, max_count);
 
+    /* Enough space for a R_readdir header: size[4] Rreaddir tag[2] count[4] */
+    if (max_count > s->msize - 11) {
+        max_count = s->msize - 11;
+        warn_report_once(
+            "9p: bad client: T_readdir with count > msize - 11"
+        );
+    }
+
     fidp = get_fid(pdu, fid);
     if (fidp == NULL) {
         retval = -EINVAL;
-- 
2.21.1