Fix memory corruption in PMAP_CALLIT code

Commit Message

Olaf Kirch Aug. 10, 2015, 1:02 p.m. UTC

From 06f7ebb1dade2f0dbf872ea2bedf17cff4734bdd Mon Sep 17 00:00:00 2001
From: Olaf Kirch <[e-mail]>
Date: Thu, 6 Aug 2015 16:27:20 +0200
Subject: [PATCH] Fix memory corruption in PMAP_CALLIT code

We have seen occasional reports of a commercial security scanner 
triggering crashes in rpcbind, but these were fairly elusive.

Now we finally got a usable core file showing that rpcbind crashed in 
svc_dodestroy when trying to free a corrupted xprt->xp_netid pointer. 
Closer inspection suggested that the pointer variable actually 
contained a sockaddr_in.

Here's how I think the memory corruption happens:

 - A PMAP_CALLIT call comes in on IPv4 UDP

 - rpcbind duplicates the caller's address to a netbuf and stores
	it in FINFO[0].caller_addr. caller_addr->buf now points to a
    memory region A with a size of 16 bytes

 - rpcbind forwards the call to the local service, receives a reply

 - when processing the reply, it does this in xprt_set_caller:
     xprt->xp_rtaddr = *FINFO[0].caller_addr
   where xprt is the UDP transport on which it received the
   PMAP_CALLIT request.

   It sends out the reply, and then frees the netbuf caller_addr and
   caller_addr.buf.
   However, it does not clear xp_rtaddr, so xp_rtaddr.buf now refers
   to memory region A, which is free.

 - When the next call comes in on the UDP/IPv4 socket, svc_dg_recv
   will be called, which will set xp_rtaddr to the client's address.
   It will reuse the buffer inside xp_rtaddr, ie it will write a
   sockaddr_in to region A.

So, this explains how memory gets corrupted. Here's why that 
eventually lead to a crash in svc_dodestroy.

Some time down the road, an incoming TCP connection is accepted,
allocating a fresh SVCXPRT. The memory region A is inside the
new SVCXPRT

 - While processing the TCP call, another UDP call comes in, again
   overwriting region A with the client's address

 - TCP client closes connection. In svc_destroy, we now trip over
   the garbage left in region A

The fix seems to be to make xprt_set_caller copy the caller's address 
to xprt->xp_rtaddr using __rpc_set_netbuf rather than just overwrite 
the netbuf.

Signed-off-by: Olaf Kirch <[e-mail]>
---
 src/rpcb_svc_com.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

 }

Patch

diff --git a/src/rpcb_svc_com.c b/src/rpcb_svc_com.c
index ff9ce6b..79fb3f5 100644
--- a/src/rpcb_svc_com.c
+++ b/src/rpcb_svc_com.c
@@ -1186,9 +1186,10 @@  check_rmtcalls(struct pollfd *pfds, int nfds)
 static void
 xprt_set_caller(SVCXPRT *xprt, struct finfo *fi)
 {
+	const struct netbuf *caller = fi->caller_addr;
 	u_int32_t *xidp;
 
-	*(svc_getrpccaller(xprt)) = *(fi->caller_addr);
+	__rpc_set_netbuf(svc_getrpccaller(xprt), caller->buf, caller-
>len);
 	xidp = __rpcb_get_dg_xidp(xprt);
 	*xidp = fi->caller_xid;