Date: Mon, 15 Sep 1997 16:53:29 +1000 (EST) From: Darren Reed <darrenr@cyber.com.au> To: security@freebsd.org Subject: OpenBSD Security Advisory: BSD I/O Signals (fwd) Message-ID: <199709150653.QAA05056@plum.cyber.com.au>
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In some mail I received from tqbf@rdist.org, sie wrote
> From tech-security-owner-darrenr=cyber.com.au@NetBSD.ORG Mon Sep 15 16:10:11 1997
> Message-ID: <19970915060810.20028.qmail@smtp.enteract.com>
> From: tqbf@rdist.org
> Subject: OpenBSD Security Advisory: BSD I/O Signals
> To: tech-security@NetBSD.ORG
> Date: Mon, 15 Sep 1997 01:08:10 -0500 (CDT)
> Reply-To: tqbf@enteract.com
> X-Mailer: ELM [version 2.4ME+ PL31 (25)]
> MIME-Version: 1.0
> Content-Type: text/plain; charset=US-ASCII
> Content-Transfer-Encoding: 7bit
> Sender: tech-security-owner@NetBSD.ORG
> Precedence: list
> Delivered-To: tech-security@NetBSD.ORG
>
> ----------------------------------------------------------------------------
>
> OpenBSD Security Advisory
>
> September 15, 1997
>
> Vulnerability in I/O Signal Handling
>
> ----------------------------------------------------------------------------
>
> SYNOPSIS
>
> A vulnerability discovered in the 4.4BSD kernel allows unprivileged users
> to send certain signals to arbitrary processes on the system. Depending on
> the operating system and targeted program, this may allow users to kill
> off processes or disrupt the operation of certain programs.
>
> ----------------------------------------------------------------------------
>
> AFFECTED SYSTEMS
>
> This vulnerability has been tested on all available 4.4BSD-based operating
> systems, including BSDI, NetBSD, OpenBSD, and FreeBSD, in their most
> recent release revisions. Additionally, this problem is known to affect
> SGI IRIX, and may affect other operating systems as well.
>
> ----------------------------------------------------------------------------
>
> DETAILS
>
> Certain programs implemented in Unix operating systems make use of a
> facility called "asynchronous I/O" to handle multiple tasks
> simultaneously. Asynchronous I/O allows a process to be notified whenever
> it has data that needs to be read from an input source; the kernel
> notifies the process using a signal.
>
> Asynchronous I/O is enabled on a descriptor using the fcntl() system call;
> a descriptor with the O_ASYNC flag set will cause a signal to be sent
> whenever there is data available to be read from it. Additionally, using
> the FIOASYNC ioctl(), asynchronous notification can be enabled on a
> descriptor.
>
> In cases where multiple processes are used in an application, it becomes
> useful to allow a descriptor to notify other processes as well. This is
> accomplished by use of another fcntl() operation, F_SETOWN, as well as an
> ioctl, FIOSETOWN (certain devices also provide an interface to this
> facility with the TIOCSPGRP ioctl). These operations allow a program to
> set the process or process group that will receive signal notification of
> pending I/O.
>
> A lack of checking in the code that handles these operations allows a
> program to specify an arbitrary process ID when using a socket or device
> file descriptor. By setting the recipient of signal notification to a
> process that is not owned by the program, the kernel can be tricked into
> signalling arbitrary programs.
>
> Additionally, vulnerable kernels do not keep track of the credentials
> associated with a process when determining whether to send I/O signals;
> because of this, it is possible to specify the PID of a process that is
> owned by an attacker, and then destroy that process and wait for it's PID
> to be re-used. The new process occupying that PID can then be signalled by
> the attacker, regardless of it's owner.
>
> It's important to note that operating systems that check credentials when
> a program attempts to set the PID for I/O notification (thus evading part
> of this vulnerability) may still be vulnerable to the latter problem
> (process ID re-use), if credentials aren't checked at signal delivery
> time. We recommend that concerned parties contact their operating system
> vendors or support channels to verify their vulnerability status.
>
> ----------------------------------------------------------------------------
>
> TECHNICAL DETAILS
>
> This vulnerability exists due to a lack of credential checking in the
> kernel when setting the recipient of I/O notification. BSD-based
> kernels maintain this information in descriptor-specific data structures
> unrelated to the process table; the vulnerability discussed here involves
> sockets, which maintain the signal recipient in the per-descriptor
> "socket" structure, although certain devices provide similar facilities
> and vulnerabilities.
>
> On 4.4BSD systems, the signal normally sent to inform a process of pending
> I/O is SIGIO. The default disposition of the SIGIO signal is "ignore" -
> thus, most processes are unaffected by this vulnerability. However,
> certain programs explicitly catch SIGIO in order to use asynchronous I/O;
> these programs can be disrupted by sending stray SIGIO's.
>
> The process notification information is also used to determine the process
> that receives notification of out-of-band data on a socket. The same
> vulnerability applies, this time by setting the process to notify and then
> sending a message with the MSG_OOB flag set; the targetted process will
> receive a SIGURG signal. Certain network daemons (ftpd, for instance) can
> be disrupted by being sent a stray SIGURG signal when no data is available
> for reading.
>
> The problem is more serious on vulnerable System V operating systems; in
> many cases, SIGIO is the equivalent of SIGPOLL, and the default
> disposition of that signal is "terminate process". On SGI's IRIX operating
> system, exploitation of this vulnerability can kill any process on the
> system.
>
> In addition to being an extremely potent denial of service attack,
> surgical application of this vulnerability can be used to compromise the
> system - for example, a process holding a bound address (NFS port 2049,
> for instance) can be killed off and it's port stolen; this can be used to
> steal NFS file handles.
>
> In addition to sockets, some devices also provide facilities for
> notification of pending I/O; examples include the "log", "tun", and "bpf".
> The BPF and tunnel devices are of minimal concern, as they are not
> typically accessible by arbitrary users (although BPF is interesting in
> that it will allow the owner of a bpf-associated descriptor to choose an
> arbitrary signal to send, including SIGSTOP).
>
> Unfortunately, the log device is normally accessible by users, and can be
> used to perform the same attack as sockets allow. It's also worth noting
> that the interface that allows programs to set the process to receive
> notification of I/O on the log device renders the legitimate purpose of
> this facility totally unreliable; unrelated programs can seize control of
> the asynchronous I/O notification on the log device, causing programs that
> rely on it to fail. The provided patches do not attempt to resolve this
> problem.
>
> ----------------------------------------------------------------------------
>
> RESOLUTION
>
> This is a kernel problem that can only be fixed by patching the
> problematic system code. Patches for the OpenBSD operating system are
> provided in this advisory; FreeBSD is known to be working on a similar
> resolution.
>
> The provided OpenBSD patch causes the kernel to keep track of the
> credentials of the process associated with an I/O object; the credentials
> are checked whenever I/O notification will occur, and therefore resolve
> both the F_SETOWN and PID-reuse problems. Device drivers that present an
> interface to I/O notification must be modified to check credentials when
> the TIOCSPGRP (or equivalent) ioctl() is used to set notificatio PID; the
> OpenBSD patch resolves all currently known occurances of this in that
> operating system.
>
> ----------------------------------------------------------------------------
>
> CREDITS
>
> This vulnerability is believed to have been discovered originally by Alan
> Peakall. Documentation and testing of this problem was conducted by Theo
> de Raadt and the OpenBSD development team; SGI information was obtained
> from Timothy Newsham.
>
> The OpenBSD patch for this vulnerability was written in a caffeinated haze
> by Theo de Raadt of the OpenBSD project.
>
> The developers at OpenBSD would like to thank Perry Metzger for his
> continuous support of their work.
>
> ----------------------------------------------------------------------------
>
> OPENBSD PATCH
>
> Index: sys/signalvar.h
> ===================================================================
> RCS file: /cvs/src/sys/sys/signalvar.h,v
> retrieving revision 1.6
> retrieving revision 1.7
> diff -u -r1.6 -r1.7
> --- signalvar.h 1997/02/01 21:49:36 1.6
> +++ signalvar.h 1997/08/31 20:42:01 1.7
> @@ -156,6 +156,7 @@
> int coredump __P((struct proc *p));
> void execsigs __P((struct proc *p));
> void gsignal __P((int pgid, int sig));
> +void csignal __P((pid_t pgid, int signum, uid_t uid, uid_t euid));
> int issignal __P((struct proc *p));
> void pgsignal __P((struct pgrp *pgrp, int sig, int checkctty));
> void postsig __P((int sig));
> Index: sys/socketvar.h
> ===================================================================
> RCS file: /cvs/src/sys/sys/socketvar.h,v
> retrieving revision 1.10
> retrieving revision 1.11
> diff -u -r1.10 -r1.11
> --- socketvar.h 1997/02/28 04:04:13 1.10
> +++ socketvar.h 1997/08/31 20:42:02 1.11
> @@ -71,6 +71,8 @@
> short so_timeo; /* connection timeout */
> u_short so_error; /* error affecting connection */
> pid_t so_pgid; /* pgid for signals */
> + uid_t so_siguid; /* uid of process who set so_pgid */
> + uid_t so_sigeuid; /* euid of process who set so_pgid */
> u_long so_oobmark; /* chars to oob mark */
> /*
> * Variables for socket buffering.
> Index: kern/kern_descrip.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_descrip.c,v
> retrieving revision 1.13
> retrieving revision 1.14
> diff -u -r1.13 -r1.14
> --- kern_descrip.c 1997/08/21 05:17:37 1.13
> +++ kern_descrip.c 1997/08/31 20:42:15 1.14
> @@ -55,6 +55,7 @@
> #include <sys/fcntl.h>
> #include <sys/malloc.h>
> #include <sys/syslog.h>
> +#include <sys/ucred.h>
> #include <sys/unistd.h>
> #include <sys/resourcevar.h>
> #include <sys/conf.h>
> @@ -251,8 +252,11 @@
>
> case F_SETOWN:
> if (fp->f_type == DTYPE_SOCKET) {
> - ((struct socket *)fp->f_data)->so_pgid =
> - (long)SCARG(uap, arg);
> + struct socket *so = (struct socket *)fp->f_data;
> +
> + so->so_pgid = (long)SCARG(uap, arg);
> + so->so_siguid = p->p_cred->p_ruid;
> + so->so_sigeuid = p->p_ucred->cr_uid;
> return (0);
> }
> if ((long)SCARG(uap, arg) <= 0) {
> Index: kern/kern_sig.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/kern_sig.c,v
> retrieving revision 1.16
> retrieving revision 1.17
> diff -u -r1.16 -r1.17
> --- kern_sig.c 1997/02/01 21:49:41 1.16
> +++ kern_sig.c 1997/08/31 20:42:18 1.17
> @@ -481,6 +481,46 @@
> }
> }
> return (nfound ? 0 : ESRCH);
> +}
> +
> +#define CANDELIVER(uid, euid, p) \
> + (euid == 0 || \
> + (uid) == (p)->p_cred->p_ruid || \
> + (uid) == (p)->p_cred->p_svuid || \
> + (uid) == (p)->p_ucred->cr_uid || \
> + (euid) == (p)->p_cred->p_ruid || \
> + (euid) == (p)->p_cred->p_svuid || \
> + (euid) == (p)->p_ucred->cr_uid)
> +
> +/*
> + * Deliver signum to pgid, but first check uid/euid against each
> + * process and see if it is permitted.
> + */
> +void
> +csignal(pgid, signum, uid, euid)
> + pid_t pgid;
> + int signum;
> + uid_t uid, euid;
> +{
> + struct pgrp *pgrp;
> + struct proc *p;
> +
> + if (pgid == 0)
> + return;
> + if (pgid < 0) {
> + pgid = -pgid;
> + if ((pgrp = pgfind(pgid)) == NULL)
> + return;
> + for (p = pgrp->pg_members.lh_first; p;
> + p = p->p_pglist.le_next)
> + if (CANDELIVER(uid, euid, p))
> + psignal(p, signum);
> + } else {
> + if ((p = pfind(pgid)) == NULL)
> + return;
> + if (CANDELIVER(uid, euid, p))
> + psignal(p, signum);
> + }
> }
>
> /*
> Index: kern/subr_log.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/subr_log.c,v
> retrieving revision 1.3
> retrieving revision 1.4
> diff -u -r1.3 -r1.4
> --- subr_log.c 1996/04/21 22:27:17 1.3
> +++ subr_log.c 1997/08/31 20:42:20 1.4
> @@ -60,6 +60,8 @@
> int sc_state; /* see above for possibilities */
> struct selinfo sc_selp; /* process waiting on select call */
> int sc_pgid; /* process/group for async I/O */
> + uid_t sc_siguid; /* uid for process that set sc_pgid */
> + uid_t sc_sigeuid; /* euid for process that set sc_pgid */
> } logsoftc;
>
> int log_open; /* also used in log() */
> @@ -179,17 +181,12 @@
> void
> logwakeup()
> {
> - struct proc *p;
> -
> if (!log_open)
> return;
> selwakeup(&logsoftc.sc_selp);
> - if (logsoftc.sc_state & LOG_ASYNC) {
> - if (logsoftc.sc_pgid < 0)
> - gsignal(-logsoftc.sc_pgid, SIGIO);
> - else if ((p = pfind(logsoftc.sc_pgid)) != NULL)
> - psignal(p, SIGIO);
> - }
> + if (logsoftc.sc_state & LOG_ASYNC)
> + csignal(logsoftc.sc_pgid, SIGIO,
> + logsoftc.sc_siguid, logsoftc.sc_sigeuid);
> if (logsoftc.sc_state & LOG_RDWAIT) {
> wakeup((caddr_t)msgbufp);
> logsoftc.sc_state &= ~LOG_RDWAIT;
> @@ -232,6 +229,8 @@
>
> case TIOCSPGRP:
> logsoftc.sc_pgid = *(int *)data;
> + logsoftc.sc_siguid = p->p_cred->p_ruid;
> + logsoftc.sc_sigeuid = p->p_ucred->cr_uid;
> break;
>
> case TIOCGPGRP:
> Index: kern/sys_generic.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/sys_generic.c,v
> retrieving revision 1.7
> retrieving revision 1.8
> diff -u -r1.7 -r1.8
> --- sys_generic.c 1997/01/27 23:21:13 1.7
> +++ sys_generic.c 1997/08/31 20:42:21 1.8
> @@ -480,7 +480,11 @@
> case FIOSETOWN:
> tmp = *(int *)data;
> if (fp->f_type == DTYPE_SOCKET) {
> - ((struct socket *)fp->f_data)->so_pgid = tmp;
> + struct socket *so = (struct socket *)fp->f_data;
> +
> + so->so_pgid = tmp;
> + so->so_siguid = p->p_cred->p_ruid;
> + so->so_sigeuid = p->p_ucred->cr_uid;
> error = 0;
> break;
> }
> Index: kern/sys_socket.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/sys_socket.c,v
> retrieving revision 1.2
> retrieving revision 1.3
> diff -u -r1.2 -r1.3
> --- sys_socket.c 1997/02/24 14:19:59 1.2
> +++ sys_socket.c 1997/08/31 20:42:23 1.3
> @@ -39,6 +39,7 @@
> #include <sys/param.h>
> #include <sys/systm.h>
> #include <sys/file.h>
> +#include <sys/proc.h>
> #include <sys/mbuf.h>
> #include <sys/protosw.h>
> #include <sys/socket.h>
> @@ -112,6 +113,8 @@
>
> case SIOCSPGRP:
> so->so_pgid = *(int *)data;
> + so->so_siguid = p->p_cred->p_ruid;
> + so->so_sigeuid = p->p_ucred->cr_uid;
> return (0);
>
> case SIOCGPGRP:
> Index: kern/uipc_socket.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/uipc_socket.c,v
> retrieving revision 1.15
> retrieving revision 1.17
> diff -u -r1.15 -r1.17
> --- uipc_socket.c 1997/06/29 18:14:35 1.15
> +++ uipc_socket.c 1997/08/31 20:42:24 1.17
> @@ -1058,11 +1060,6 @@
> sohasoutofband(so)
> register struct socket *so;
> {
> - struct proc *p;
> -
> - if (so->so_pgid < 0)
> - gsignal(-so->so_pgid, SIGURG);
> - else if (so->so_pgid > 0 && (p = pfind(so->so_pgid)) != 0)
> - psignal(p, SIGURG);
> + csignal(so->so_pgid, SIGURG, so->so_siguid, so->so_sigeuid);
> selwakeup(&so->so_rcv.sb_sel);
> }
> Index: kern/uipc_socket2.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/uipc_socket2.c,v
> retrieving revision 1.5
> retrieving revision 1.6
> diff -u -r1.5 -r1.6
> --- uipc_socket2.c 1997/02/21 08:45:00 1.5
> +++ uipc_socket2.c 1997/08/31 20:42:26 1.6
> @@ -315,20 +315,14 @@
> register struct socket *so;
> register struct sockbuf *sb;
> {
> - struct proc *p;
> -
> selwakeup(&sb->sb_sel);
> sb->sb_flags &= ~SB_SEL;
> if (sb->sb_flags & SB_WAIT) {
> sb->sb_flags &= ~SB_WAIT;
> wakeup((caddr_t)&sb->sb_cc);
> }
> - if (so->so_state & SS_ASYNC) {
> - if (so->so_pgid < 0)
> - gsignal(-so->so_pgid, SIGIO);
> - else if (so->so_pgid > 0 && (p = pfind(so->so_pgid)) != 0)
> - psignal(p, SIGIO);
> - }
> + if (so->so_state & SS_ASYNC)
> + csignal(so->so_pgid, SIGIO, so->so_siguid, so->so_sigeuid);
> }
>
> /*
> Index: net/bpf.c
> ===================================================================
> RCS file: /cvs/src/sys/net/bpf.c,v
> retrieving revision 1.9
> retrieving revision 1.10
> diff -u -r1.9 -r1.10
> --- bpf.c 1997/03/17 16:29:37 1.9
> +++ bpf.c 1997/08/31 20:42:29 1.10
> @@ -522,14 +522,10 @@
> bpf_wakeup(d)
> register struct bpf_d *d;
> {
> - struct proc *p;
> -
> wakeup((caddr_t)d);
> if (d->bd_async && d->bd_sig)
> - if (d->bd_pgid > 0)
> - gsignal (d->bd_pgid, d->bd_sig);
> - else if ((p = pfind (-d->bd_pgid)) != NULL)
> - psignal (p, d->bd_sig);
> + csignal(d->bd_pgid, d->bd_sig,
> + d->bd_siguid, d->bd_sigeuid);
>
> #if BSD >= 199103
> selwakeup(&d->bd_sel);
> @@ -822,6 +818,8 @@
> */
> case TIOCSPGRP: /* Process or group to send signals to */
> d->bd_pgid = *(int *)addr;
> + d->bd_siguid = p->p_cred->p_ruid;
> + d->bd_sigeuid = p->p_ucred->cr_uid;
> break;
>
> case TIOCGPGRP:
> Index: net/bpfdesc.h
> ===================================================================
> RCS file: /cvs/src/sys/net/bpfdesc.h,v
> retrieving revision 1.2
> retrieving revision 1.3
> diff -u -r1.2 -r1.3
> --- bpfdesc.h 1997/02/24 13:33:56 1.2
> +++ bpfdesc.h 1997/08/31 20:42:30 1.3
> @@ -77,6 +77,8 @@
> int bd_async; /* non-zero if packet reception should generate signal */
> int bd_sig; /* signal to send upon packet reception */
> pid_t bd_pgid; /* process or group id for signal */
> + uid_t bd_siguid; /* uid for process that set pgid */
> + uid_t bd_sigeuid; /* euid for process that set pgid */
> #if BSD < 199103
> u_char bd_selcoll; /* true if selects collide */
> int bd_timedout;
> Index: net/if_tun.c
> ===================================================================
> RCS file: /cvs/src/sys/net/if_tun.c,v
> retrieving revision 1.19
> retrieving revision 1.20
> diff -u -r1.19 -r1.20
> --- if_tun.c 1997/07/29 07:18:20 1.19
> +++ if_tun.c 1997/08/31 20:42:32 1.20
> @@ -84,7 +84,9 @@
> struct tun_softc {
> u_short tun_flags; /* misc flags */
> struct ifnet tun_if; /* the interface */
> - int tun_pgrp; /* the process group - if any */
> + pid_t tun_pgid; /* the process group - if any */
> + uid_t tun_siguid; /* uid for process that set tun_pgid */
> + uid_t tun_sigeuid; /* euid for process that set tun_pgid */
> struct selinfo tun_rsel; /* read select */
> struct selinfo tun_wsel; /* write select (not used) */
> };
> @@ -228,7 +230,7 @@
> }
> splx(s);
> }
> - tp->tun_pgrp = 0;
> + tp->tun_pgid = 0;
> selwakeup(&tp->tun_rsel);
>
> TUNDEBUG(("%s: closed\n", ifp->if_xname));
> @@ -331,7 +333,6 @@
> {
> struct tun_softc *tp = ifp->if_softc;
> struct tunnel_header *th;
> - struct proc *p;
> int s;
>
> TUNDEBUG(("%s: tun_output\n", ifp->if_xname));
> @@ -371,12 +372,9 @@
> tp->tun_flags &= ~TUN_RWAIT;
> wakeup((caddr_t)tp);
> }
> - if (tp->tun_flags & TUN_ASYNC && tp->tun_pgrp) {
> - if (tp->tun_pgrp > 0)
> - gsignal(tp->tun_pgrp, SIGIO);
> - else if ((p = pfind(-tp->tun_pgrp)) != NULL)
> - psignal(p, SIGIO);
> - }
> + if (tp->tun_flags & TUN_ASYNC && tp->tun_pgid)
> + csignal(tp->tun_pgid, SIGIO,
> + tp->tun_siguid, tp->tun_sigeuid);
> selwakeup(&tp->tun_rsel);
> return 0;
> }
> @@ -446,10 +444,12 @@
> splx(s);
> break;
> case TIOCSPGRP:
> - tp->tun_pgrp = *(int *)data;
> + tp->tun_pgid = *(int *)data;
> + tp->tun_siguid = p->p_cred->p_ruid;
> + tp->tun_sigeuid = p->p_ucred->cr_uid;
> break;
> case TIOCGPGRP:
> - *(int *)data = tp->tun_pgrp;
> + *(int *)data = tp->tun_pgid;
> break;
> default:
> splx(s);
>
>
>
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