Changes to resolve races in bpfread() and catchpacket() that, at worst,

cause kernel panics.

Add a flag to the bpf descriptor to indicate whether the hold buffer
is in use. In bpfread(), set the "hold buffer in use" flag before
dropping the descriptor lock during the call to bpf_uiomove().
Everywhere else the hold buffer is used or changed, wait while
the hold buffer is in use by bpfread(). Add a KASSERT in bpfread()
after re-acquiring the descriptor lock to assist uncovering any
additional hold buffer races.

sys/net/bpf.c

@@ -819,6 +819,7 @@ bpfopen(struct cdev *dev, int flags, int fmt, struct thread *td)
 	 * particular buffer method.
 	 */
 	bpf_buffer_init(d);
+	d->bd_hbuf_in_use = 0;
 	d->bd_bufmode = BPF_BUFMODE_BUFFER;
 	d->bd_sig = SIGIO;
 	d->bd_direction = BPF_D_INOUT;
@@ -872,6 +873,9 @@ bpfread(struct cdev *dev, struct uio *uio, int ioflag)
 		callout_stop(&d->bd_callout);
 	timed_out = (d->bd_state == BPF_TIMED_OUT);
 	d->bd_state = BPF_IDLE;
+	while (d->bd_hbuf_in_use)
+		mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
+		    PRINET|PCATCH, "bd_hbuf", 0);
 	/*
 	 * If the hold buffer is empty, then do a timed sleep, which
 	 * ends when the timeout expires or when enough packets
@@ -940,27 +944,27 @@ bpfread(struct cdev *dev, struct uio *uio, int ioflag)
 	/*
 	 * At this point, we know we have something in the hold slot.
 	 */
+	d->bd_hbuf_in_use = 1;
 	BPFD_UNLOCK(d);
 
 	/*
 	 * Move data from hold buffer into user space.
 	 * We know the entire buffer is transferred since
 	 * we checked above that the read buffer is bpf_bufsize bytes.
-	 *
-	 * XXXRW: More synchronization needed here: what if a second thread
-	 * issues a read on the same fd at the same time?  Don't want this
-	 * getting invalidated.
+  	 *
+	 * We do not have to worry about simultaneous reads because
+	 * we waited for sole access to the hold buffer above.
 	 */
 	error = bpf_uiomove(d, d->bd_hbuf, d->bd_hlen, uio);
 
 	BPFD_LOCK(d);
-	if (d->bd_hbuf != NULL) {
-		/* Free the hold buffer only if it is still valid. */
-		d->bd_fbuf = d->bd_hbuf;
-		d->bd_hbuf = NULL;
-		d->bd_hlen = 0;
-		bpf_buf_reclaimed(d);
-	}
+	KASSERT(d->bd_hbuf != NULL, ("bpfread: lost bd_hbuf"));
+	d->bd_fbuf = d->bd_hbuf;
+	d->bd_hbuf = NULL;
+	d->bd_hlen = 0;
+	bpf_buf_reclaimed(d);
+	d->bd_hbuf_in_use = 0;
+	wakeup(&d->bd_hbuf_in_use);
 	BPFD_UNLOCK(d);
 
 	return (error);
@@ -1114,6 +1118,9 @@ reset_d(struct bpf_d *d)
 
 	BPFD_LOCK_ASSERT(d);
 
+	while (d->bd_hbuf_in_use)
+		mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock, PRINET,
+		    "bd_hbuf", 0);
 	if ((d->bd_hbuf != NULL) &&
 	    (d->bd_bufmode != BPF_BUFMODE_ZBUF || bpf_canfreebuf(d))) {
 		/* Free the hold buffer. */
@@ -1254,6 +1261,9 @@ bpfioctl(struct cdev *dev, u_long cmd, caddr_t addr, int flags,
 
 			BPFD_LOCK(d);
 			n = d->bd_slen;
+			while (d->bd_hbuf_in_use)
+				mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
+				    PRINET, "bd_hbuf", 0);
 			if (d->bd_hbuf)
 				n += d->bd_hlen;
 			BPFD_UNLOCK(d);
@@ -1967,6 +1977,9 @@ filt_bpfread(struct knote *kn, long hint)
 	ready = bpf_ready(d);
 	if (ready) {
 		kn->kn_data = d->bd_slen;
+		while (d->bd_hbuf_in_use)
+			mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
+			    PRINET, "bd_hbuf", 0);
 		if (d->bd_hbuf)
 			kn->kn_data += d->bd_hlen;
 	} else if (d->bd_rtout > 0 && d->bd_state == BPF_IDLE) {
@@ -2299,6 +2312,9 @@ catchpacket(struct bpf_d *d, u_char *pkt, u_int pktlen, u_int snaplen,
 	 * spot to do it.
 	 */
 	if (d->bd_fbuf == NULL && bpf_canfreebuf(d)) {
+		while (d->bd_hbuf_in_use)
+			mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
+			    PRINET, "bd_hbuf", 0);
 		d->bd_fbuf = d->bd_hbuf;
 		d->bd_hbuf = NULL;
 		d->bd_hlen = 0;
@@ -2341,6 +2357,9 @@ catchpacket(struct bpf_d *d, u_char *pkt, u_int pktlen, u_int snaplen,
 			++d->bd_dcount;
 			return;
 		}
+		while (d->bd_hbuf_in_use)
+			mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
+			    PRINET, "bd_hbuf", 0);
 		ROTATE_BUFFERS(d);
 		do_wakeup = 1;
 		curlen = 0;

sys/net/bpf.h

@@ -1235,7 +1235,8 @@ SYSCTL_DECL(_net_bpf);
 /*
  * Rotate the packet buffers in descriptor d.  Move the store buffer into the
  * hold slot, and the free buffer ino the store slot.  Zero the length of the
- * new store buffer.  Descriptor lock should be held.
+ * new store buffer.  Descriptor lock should be held. Hold buffer must
+ * not be marked "in use".
  */
 #define	ROTATE_BUFFERS(d)	do {					\
 	(d)->bd_hbuf = (d)->bd_sbuf;					\

sys/net/bpf_buffer.c

@@ -79,6 +79,8 @@ __FBSDID("$FreeBSD$");
 #include <net/bpf_buffer.h>
 #include <net/bpfdesc.h>
 
+#define PRINET  26			/* interruptible */
+
 /*
  * Implement historical kernel memory buffering model for BPF: two malloc(9)
  * kernel buffers are hung off of the descriptor.  The size is fixed prior to
@@ -189,6 +191,9 @@ bpf_buffer_ioctl_sblen(struct bpf_d *d, u_int *i)
 		return (EINVAL);
 	}
 
+	while (d->bd_hbuf_in_use)
+		mtx_sleep(&d->bd_hbuf_in_use, &d->bd_lock,
+		    PRINET, "bd_hbuf", 0);
 	/* Free old buffers if set */
 	if (d->bd_fbuf != NULL)
 		free(d->bd_fbuf, M_BPF);

sys/net/bpfdesc.h

@@ -63,6 +63,7 @@ struct bpf_d {
 	caddr_t		bd_sbuf;	/* store slot */
 	caddr_t		bd_hbuf;	/* hold slot */
 	caddr_t		bd_fbuf;	/* free slot */
+	int		bd_hbuf_in_use;	/* don't rotate buffers */
 	int 		bd_slen;	/* current length of store buffer */
 	int 		bd_hlen;	/* current length of hold buffer */