Re: [PATCH v2 1/1] Fix DoS vulnerability in libtirpc

[Steve Dickson <steved@redhat.com>]

On 8/10/21 8:08 PM, Dai Ngo wrote:
> Currently svc_run does not handle poll timeout and rendezvous_request
> does not handle EMFILE error returned from accept(2 as it used to.
> These two missing functionality were removed by commit b2c9430f46c4.
> 
> The effect of not handling poll timeout allows idle TCP conections
> to remain ESTABLISHED indefinitely. When the number of connections
> reaches the limit of the open file descriptors (ulimit -n) then
> accept(2) fails with EMFILE. Since there is no handling of EMFILE
> error this causes svc_run() to get in a tight loop calling accept(2).
> This resulting in the RPC service of svc_run is being down, it's
> no longer able to service any requests.
> 
> if [ $# -ne 3 ]; then
>          echo "$0: server dst_port conn_cnt"
>          exit
> fi
> server=$1
> dport=$2
> conn_cnt=$3
> echo "dport[$dport] server[$server] conn_cnt[$conn_cnt]"
> 
> pcnt=0
> while [ $pcnt -lt $conn_cnt ]
> do
>          nc -v --recv-only $server $dport &
>          pcnt=`expr $pcnt + 1`
> done
> 
> RPC service rpcbind, statd and mountd are effected by this
> problem.
> 
> Fix by enhancing rendezvous_request to keep the number of
> SVCXPRT conections to 4/5 of the size of the file descriptor
> table. When this thresold is reached, it destroys the idle
> TCP connections or destroys the least active connection if
> no idle connnction was found.
> 
> Fixes: 44bf15b8 rpcbind: don't use obsolete svc_fdset interface of libtirpc
> Signed-off-by: dai.ngo@oracle.com
Committed... (tag: libtirpc-1-3-3-rc1)

steved.
> ---
>   src/svc.c    | 17 +++++++++++++-
>   src/svc_vc.c | 62 +++++++++++++++++++++++++++++++++++++++++++++++++++-
>   2 files changed, 77 insertions(+), 2 deletions(-)
> 
> diff --git a/src/svc.c b/src/svc.c
> index 6db164bbd76b..3a8709fe375c 100644
> --- a/src/svc.c
> +++ b/src/svc.c
> @@ -57,7 +57,7 @@
>   
>   #define max(a, b) (a > b ? a : b)
>   
> -static SVCXPRT **__svc_xports;
> +SVCXPRT **__svc_xports;
>   int __svc_maxrec;
>   
>   /*
> @@ -194,6 +194,21 @@ __xprt_do_unregister (xprt, dolock)
>       rwlock_unlock (&svc_fd_lock);
>   }
>   
> +int
> +svc_open_fds()
> +{
> +	int ix;
> +	int nfds = 0;
> +
> +	rwlock_rdlock (&svc_fd_lock);
> +	for (ix = 0; ix < svc_max_pollfd; ++ix) {
> +		if (svc_pollfd[ix].fd != -1)
> +			nfds++;
> +	}
> +	rwlock_unlock (&svc_fd_lock);
> +	return (nfds);
> +}
> +
>   /*
>    * Add a service program to the callout list.
>    * The dispatch routine will be called when a rpc request for this
> diff --git a/src/svc_vc.c b/src/svc_vc.c
> index f1d9f001fcdc..3dc8a75787e1 100644
> --- a/src/svc_vc.c
> +++ b/src/svc_vc.c
> @@ -64,6 +64,8 @@
>   
>   
>   extern rwlock_t svc_fd_lock;
> +extern SVCXPRT **__svc_xports;
> +extern int svc_open_fds();
>   
>   static SVCXPRT *makefd_xprt(int, u_int, u_int);
>   static bool_t rendezvous_request(SVCXPRT *, struct rpc_msg *);
> @@ -82,6 +84,7 @@ static void svc_vc_ops(SVCXPRT *);
>   static bool_t svc_vc_control(SVCXPRT *xprt, const u_int rq, void *in);
>   static bool_t svc_vc_rendezvous_control (SVCXPRT *xprt, const u_int rq,
>   				   	     void *in);
> +static int __svc_destroy_idle(int timeout);
>   
>   struct cf_rendezvous { /* kept in xprt->xp_p1 for rendezvouser */
>   	u_int sendsize;
> @@ -313,13 +316,14 @@ done:
>   	return (xprt);
>   }
>   
> +
>   /*ARGSUSED*/
>   static bool_t
>   rendezvous_request(xprt, msg)
>   	SVCXPRT *xprt;
>   	struct rpc_msg *msg;
>   {
> -	int sock, flags;
> +	int sock, flags, nfds, cnt;
>   	struct cf_rendezvous *r;
>   	struct cf_conn *cd;
>   	struct sockaddr_storage addr;
> @@ -379,6 +383,16 @@ again:
>   
>   	gettimeofday(&cd->last_recv_time, NULL);
>   
> +	nfds = svc_open_fds();
> +	if (nfds >= (_rpc_dtablesize() / 5) * 4) {
> +		/* destroy idle connections */
> +		cnt = __svc_destroy_idle(15);
> +		if (cnt == 0) {
> +			/* destroy least active */
> +			__svc_destroy_idle(0);
> +		}
> +	}
> +
>   	return (FALSE); /* there is never an rpc msg to be processed */
>   }
>   
> @@ -820,3 +834,49 @@ __svc_clean_idle(fd_set *fds, int timeout, bool_t cleanblock)
>   {
>   	return FALSE;
>   }
> +
> +static int
> +__svc_destroy_idle(int timeout)
> +{
> +	int i, ncleaned = 0;
> +	SVCXPRT *xprt, *least_active;
> +	struct timeval tv, tdiff, tmax;
> +	struct cf_conn *cd;
> +
> +	gettimeofday(&tv, NULL);
> +	tmax.tv_sec = tmax.tv_usec = 0;
> +	least_active = NULL;
> +	rwlock_wrlock(&svc_fd_lock);
> +
> +	for (i = 0; i <= svc_max_pollfd; i++) {
> +		if (svc_pollfd[i].fd == -1)
> +			continue;
> +		xprt = __svc_xports[i];
> +		if (xprt == NULL || xprt->xp_ops == NULL ||
> +			xprt->xp_ops->xp_recv != svc_vc_recv)
> +			continue;
> +		cd = (struct cf_conn *)xprt->xp_p1;
> +		if (!cd->nonblock)
> +			continue;
> +		if (timeout == 0) {
> +			timersub(&tv, &cd->last_recv_time, &tdiff);
> +			if (timercmp(&tdiff, &tmax, >)) {
> +				tmax = tdiff;
> +				least_active = xprt;
> +			}
> +			continue;
> +		}
> +		if (tv.tv_sec - cd->last_recv_time.tv_sec > timeout) {
> +			__xprt_unregister_unlocked(xprt);
> +			__svc_vc_dodestroy(xprt);
> +			ncleaned++;
> +		}
> +	}
> +	if (timeout == 0 && least_active != NULL) {
> +		__xprt_unregister_unlocked(least_active);
> +		__svc_vc_dodestroy(least_active);
> +		ncleaned++;
> +	}
> +	rwlock_unlock(&svc_fd_lock);
> +	return (ncleaned);
> +}
>