Fix a file descriptor race I reintroduced when I split accept1() up into

kern_accept() and accept1().  If another thread closed the new file
descriptor and the first thread later got an error trying to copyout the
socket address, then it would attempt to close the wrong file object.  To
fix, add a struct file ** argument to kern_accept().  If it is non-NULL,
then on success kern_accept() will store a pointer to the new file object
there and not release any of the references.  It is up to the calling code
to drop the references appropriately (including a call to fdclose() in case
of error to safely handle the aforementioned race).  While I'm at it, go
ahead and fix the svr4 streams code to not leak the accept fd if it gets an
error trying to copyout the streams structures.

sys/compat/svr4/svr4_stream.c

@@ -1637,10 +1637,12 @@ svr4_do_getmsg(td, uap, fp)
 	struct sockaddr *sa;
 	socklen_t sasize;
 	struct svr4_strm *st;
+	struct file *afp;
 	int fl;
 
 	retval = td->td_retval;
 	error = 0;
+	afp = NULL;
 
 	FILE_LOCK_ASSERT(fp, MA_NOTOWNED);
 
@@ -1778,7 +1780,7 @@ svr4_do_getmsg(td, uap, fp)
 		 * We are after a listen, so we try to accept...
 		 */
 
-		error = kern_accept(td, uap->fd, &sa, &sasize);
+		error = kern_accept(td, uap->fd, &sa, &sasize, &afp);
 		if (error) {
 			DPRINTF(("getmsg: accept failed %d\n", error));
 			return error;
@@ -1809,6 +1811,9 @@ svr4_do_getmsg(td, uap, fp)
 			break;
 
 		default:
+			fdclose(td->td_proc->p_fd, afp, st->s_afd, td);
+			fdrop(afp, td);
+			st->s_afd = -1;
 			free(sa, M_SONAME);
 			return ENOSYS;
 		}
@@ -1914,28 +1919,37 @@ svr4_do_getmsg(td, uap, fp)
 		return EINVAL;
 	}
 
-	/* XXX: We leak the accept fd if we get an error here. */
 	if (uap->ctl) {
 		if (ctl.len > sizeof(sc))
 			ctl.len = sizeof(sc);
 		if (ctl.len != -1)
-			if ((error = copyout(&sc, ctl.buf, ctl.len)) != 0)
-				return error;
+			error = copyout(&sc, ctl.buf, ctl.len);
 
-		if ((error = copyout(&ctl, uap->ctl, sizeof(ctl))) != 0)
-			return error;
+		if (error == 0)
+			error = copyout(&ctl, uap->ctl, sizeof(ctl));
 	}
 
 	if (uap->dat) {
-		if ((error = copyout(&dat, uap->dat, sizeof(dat))) != 0)
-			return error;
+		if (error == 0)
+			error = copyout(&dat, uap->dat, sizeof(dat));
 	}
 
 	if (uap->flags) { /* XXX: Need translation */
-		if ((error = copyout(&fl, uap->flags, sizeof(fl))) != 0)
-			return error;
+		if (error == 0)
+			error = copyout(&fl, uap->flags, sizeof(fl));
 	}
 
+	if (error) {
+		if (afp) {
+			fdclose(td->td_proc->p_fd, afp, st->s_afd, td);
+			fdrop(afp, td);
+			st->s_afd = -1;
+		}
+		return (error);
+	}
+	if (afp)
+		fdrop(afp, td);
+
 	*retval = 0;
 
 #ifdef DEBUG_SVR4

sys/kern/uipc_syscalls.c

@@ -299,16 +299,17 @@ accept1(td, uap, compat)
 {
 	struct sockaddr *name;
 	socklen_t namelen;
+	struct file *fp;
 	int error;
 
 	if (uap->name == NULL)
-		return (kern_accept(td, uap->s, NULL, NULL));
+		return (kern_accept(td, uap->s, NULL, NULL, NULL));
 
 	error = copyin(uap->anamelen, &namelen, sizeof (namelen));
 	if (error)
 		return (error);
 
-	error = kern_accept(td, uap->s, &name, &namelen);
+	error = kern_accept(td, uap->s, &name, &namelen, &fp);
 
 	/*
 	 * return a namelen of zero for older code which might
@@ -332,14 +333,15 @@ accept1(td, uap, compat)
 		error = copyout(&namelen, uap->anamelen,
 		    sizeof(namelen));
 	if (error)
-		kern_close(td, td->td_retval[0]);
+		fdclose(td->td_proc->p_fd, fp, td->td_retval[0], td);
+	fdrop(fp, td);
 	free(name, M_SONAME);
 	return (error);
 }
 
 int
 kern_accept(struct thread *td, int s, struct sockaddr **name,
-    socklen_t *namelen)
+    socklen_t *namelen, struct file **fp)
 {
 	struct filedesc *fdp;
 	struct file *headfp, *nfp = NULL;
@@ -478,9 +480,17 @@ kern_accept(struct thread *td, int s, struct sockaddr **name,
 		fdclose(fdp, nfp, fd, td);
 
 	/*
-	 * Release explicitly held references before returning.
+	 * Release explicitly held references before returning.  We return
+	 * a reference on nfp to the caller on success if they request it.
 	 */
 done:
+	if (fp != NULL) {
+		if (error == 0) {
+			*fp = nfp;
+			nfp = NULL;
+		} else
+			*fp = NULL;
+	}
 	if (nfp != NULL)
 		fdrop(nfp, td);
 	fdrop(headfp, td);

sys/sys/syscallsubr.h

@@ -34,6 +34,7 @@
 #include <sys/mac.h>
 #include <sys/mount.h>
 
+struct file;
 struct itimerval;
 struct image_args;
 struct mbuf;
@@ -51,7 +52,7 @@ struct sendfile_args;
 int	kern___getcwd(struct thread *td, u_char *buf, enum uio_seg bufseg,
 	    u_int buflen);
 int	kern_accept(struct thread *td, int s, struct sockaddr **name,
-	    socklen_t *namelen);
+	    socklen_t *namelen, struct file **fp);
 int	kern_access(struct thread *td, char *path, enum uio_seg pathseg,
 	    int flags);
 int	kern_adjtime(struct thread *td, struct timeval *delta,