Fix denial of service - memory corruption.

CVE-2011-0719

Fix bug #7949 (DoS in Winbind and smbd with many file descriptors open).

All current released versions of Samba are vulnerable to
a denial of service caused by memory corruption. Range
checks on file descriptors being used in the FD_SET macro
were not present allowing stack corruption. This can cause
the Samba code to crash or to loop attempting to select
on a bad file descriptor set.

A connection to a file share, or a local account is needed
to exploit this problem, either authenticated or unauthenticated
(guest connection).

Currently we do not believe this flaw is exploitable
beyond a crash or causing the code to loop, but on the
advice of our security reviewers we are releasing fixes
in case an exploit is discovered at a later date.

diff --git a/source/client/client.c b/source/client/client.c
index 53bd9e6f9aa814f0b782effbf62604a0e07ebdcf..a9894418af6e9182ebca61fa08bb72258eaa0151 100644 (file)
--- a/source/client/client.c
+++ b/source/client/client.c
@@ -4379,8 +4379,10 @@ static void readline_callback(void)
 
  again:
 
-       if (cli->fd == -1)
+       if (cli->fd < 0 || cli->fd >= FD_SETSIZE) {
+               errno = EBADF;
                return;
+       }
 
        FD_ZERO(&fds);
        FD_SET(cli->fd,&fds);

diff --git a/source/client/dnsbrowse.c b/source/client/dnsbrowse.c
index 5e3a4de9cff03afb16397a8a6ee03d0579d87326..aa2fb22ad658b1e08d02580d98d78f42122b4b8c 100644 (file)
--- a/source/client/dnsbrowse.c
+++ b/source/client/dnsbrowse.c
@@ -81,6 +81,11 @@ static void do_smb_resolve(struct mdns_smbsrv_result *browsesrv)
                        TALLOC_FREE(fdset);
                }
 
+               if (mdnsfd < 0 || mdnsfd >= FD_SETSIZE) {
+                       errno = EBADF;
+                       break;
+               }
+
                fdsetsz = howmany(mdnsfd + 1, NFDBITS) * sizeof(fd_mask);
                fdset = TALLOC_ZERO(ctx, fdsetsz);
                FD_SET(mdnsfd, fdset);
@@ -183,6 +188,13 @@ int do_smb_browse(void)
 
                fdsetsz = howmany(mdnsfd + 1, NFDBITS) * sizeof(fd_mask);
                fdset = TALLOC_ZERO(ctx, fdsetsz);
+
+               if (mdnsfd < 0 || mdnsfd >= FD_SETSIZE) {
+                       errno = EBADF;
+                       TALLOC_FREE(ctx);
+                       return 1;
+               }
+
                FD_SET(mdnsfd, fdset);
 
                tv.tv_sec = 1;

diff --git a/source/lib/events.c b/source/lib/events.c
index cd20ceb43f8fb3ab40eead77465829c3b7c8151d..2ddbab7f24a7e755ed854e30161dec7b45b08245 100644 (file)
--- a/source/lib/events.c
+++ b/source/lib/events.c
@@ -140,6 +140,11 @@ struct fd_event *event_add_fd(struct event_context *event_ctx,
 {
        struct fd_event *fde;
 
+       if (fd < 0 || fd >= FD_SETSIZE) {
+               errno = EBADF;
+               return NULL;
+       }
+
        if (!(fde = TALLOC_P(mem_ctx, struct fd_event))) {
                return NULL;
        }
@@ -190,6 +195,14 @@ bool event_add_to_select_args(struct event_context *event_ctx,
        bool ret = False;
 
        for (fde = event_ctx->fd_events; fde; fde = fde->next) {
+               if (fde->fd < 0 || fde->fd >= FD_SETSIZE) {
+                       /* We ignore here, as it shouldn't be
+                          possible to add an invalid fde->fd
+                          but we don't want FD_SET to see an
+                          invalid fd. */
+                       continue;
+               }
+
                if (fde->flags & EVENT_FD_READ) {
                        FD_SET(fde->fd, read_fds);
                        ret = True;

diff --git a/source/lib/packet.c b/source/lib/packet.c
index e0486165f34efa78ad31c4b5c76d0210b534dff9..512c7f2f8f5ffad3550e78b7a1cfb53792397bdd 100644 (file)
--- a/source/lib/packet.c
+++ b/source/lib/packet.c
@@ -106,6 +106,11 @@ NTSTATUS packet_fd_read_sync(struct packet_context *ctx)
        int res;
        fd_set r_fds;
 
+       if (ctx->fd < 0 || ctx->fd >= FD_SETSIZE) {
+               errno = EBADF;
+               return map_nt_error_from_unix(errno);
+       }
+
        FD_ZERO(&r_fds);
        FD_SET(ctx->fd, &r_fds);
 

diff --git a/source/lib/readline.c b/source/lib/readline.c
index 34867aad9e7eaccdefcf48d0047b4e932f318d25..70a82f27ab7b5abf325af0051de2480c5ddd9d23 100644 (file)
--- a/source/lib/readline.c
+++ b/source/lib/readline.c
@@ -91,6 +91,11 @@ static char *smb_readline_replacement(const char *prompt, void (*callback)(void)
                timeout.tv_sec = 5;
                timeout.tv_usec = 0;
 
+               if (fd < 0 || fd >= FD_SETSIZE) {
+                       errno = EBADF;
+                       break;
+               }
+
                FD_ZERO(&fds);
                FD_SET(fd,&fds);
 

diff --git a/source/lib/select.c b/source/lib/select.c
index c3da6a9bbaf4924a820c45ac72898ad92d321702..2d5f02c094524e8964d9fcb131ae987353804b65 100644 (file)
--- a/source/lib/select.c
+++ b/source/lib/select.c
@@ -61,6 +61,11 @@ int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, s
                if (pipe(select_pipe) == -1)
                        smb_panic("Could not create select pipe");
 
+               if (select_pipe[0] < 0 || select_pipe[0] >= FD_SETSIZE) {
+                       errno = EBADF;
+                       return -1;
+               }
+
                /*
                 * These next two lines seem to fix a bug with the Linux
                 * 2.0.x kernel (and probably other UNIXes as well) where
@@ -87,6 +92,7 @@ int sys_select(int maxfd, fd_set *readfds, fd_set *writefds, fd_set *errorfds, s
                readfds2 = &readfds_buf;
                FD_ZERO(readfds2);
        }
+
        FD_SET(select_pipe[0], readfds2);
 
        errno = 0;

diff --git a/source/lib/util_sock.c b/source/lib/util_sock.c
index 650bd1302f8ba260cf5a59125954a3f202684c8a..8aa2c97f235d543c4c84f1225f6e58e82ea25f78 100644 (file)
--- a/source/lib/util_sock.c
+++ b/source/lib/util_sock.c
@@ -960,6 +960,11 @@ NTSTATUS read_socket_with_timeout(int fd, char *buf,
        timeout.tv_usec = (long)(1000 * (time_out % 1000));
 
        for (nread=0; nread < mincnt; ) {
+               if (fd < 0 || fd >= FD_SETSIZE) {
+                       errno = EBADF;
+                       return map_nt_error_from_unix(EBADF);
+               }
+
                FD_ZERO(&fds);
                FD_SET(fd,&fds);
 
@@ -1492,7 +1497,7 @@ bool open_any_socket_out(struct sockaddr_storage *addrs, int num_addrs,
 
        for (i=0; i<num_addrs; i++) {
                sockets[i] = socket(addrs[i].ss_family, SOCK_STREAM, 0);
-               if (sockets[i] < 0)
+               if (sockets[i] < 0 || sockets[i] >= FD_SETSIZE)
                        goto done;
                set_blocking(sockets[i], false);
        }
@@ -1541,8 +1546,10 @@ bool open_any_socket_out(struct sockaddr_storage *addrs, int num_addrs,
        FD_ZERO(&r_fds);
 
        for (i=0; i<num_addrs; i++) {
-               if (sockets[i] == -1)
+               if (sockets[i] < 0 || sockets[i] >= FD_SETSIZE) {
+                       /* This cannot happen - ignore if so. */
                        continue;
+               }
                FD_SET(sockets[i], &wr_fds);
                FD_SET(sockets[i], &r_fds);
                if (sockets[i]>maxfd)

diff --git a/source/libaddns/dnssock.c b/source/libaddns/dnssock.c
index 7c8bd418e5765e5dc97ccd3e85a1c0dfb8aca1f9..f427bd59b6b144714e39cb7e57e08a3b06c41f96 100644 (file)
--- a/source/libaddns/dnssock.c
+++ b/source/libaddns/dnssock.c
@@ -218,7 +218,11 @@ static DNS_ERROR read_all(int fd, uint8 *data, size_t len)
        while (total < len) {
                ssize_t ret;
                int fd_ready;
-               
+
+               if (fd < 0 || fd >= FD_SETSIZE) {
+                       return ERROR_DNS_SOCKET_ERROR;
+               }
+
                FD_ZERO( &rfds );
                FD_SET( fd, &rfds );
 

diff --git a/source/libsmb/nmblib.c b/source/libsmb/nmblib.c
index bfe5e7b97bda22a89303e15c1be05e5e75b41b80..768e54d3da4130f311a9f961ff95d12a61f4468e 100644 (file)
--- a/source/libsmb/nmblib.c
+++ b/source/libsmb/nmblib.c
@@ -1097,6 +1097,11 @@ struct packet_struct *receive_packet(int fd,enum packet_type type,int t)
        struct timeval timeout;
        int ret;
 
+       if (fd < 0 || fd >= FD_SETSIZE) {
+               errno = EBADF;
+               return NULL;
+       }
+
        FD_ZERO(&fds);
        FD_SET(fd,&fds);
        timeout.tv_sec = t/1000;

diff --git a/source/nmbd/nmbd_packets.c b/source/nmbd/nmbd_packets.c
index 4b97819a14183a03ef4d72f92b6f5b561f364b67..03e53623e2178367a0721d8c2693afa7cb8ed63e 100644 (file)
--- a/source/nmbd/nmbd_packets.c
+++ b/source/nmbd/nmbd_packets.c
@@ -1683,7 +1683,7 @@ static bool create_listen_fdset(fd_set **ppset, int **psock_array, int *listen_n
        for (subrec = FIRST_SUBNET; subrec; subrec = NEXT_SUBNET_EXCLUDING_UNICAST(subrec))
                count++;
 
-       if((count*2) + 2 > FD_SETSIZE) {
+       if((count*2) + 2 >= FD_SETSIZE) {
                DEBUG(0,("create_listen_fdset: Too many file descriptors needed (%d). We can \
 only use %d.\n", (count*2) + 2, FD_SETSIZE));
                SAFE_FREE(pset);
@@ -1699,24 +1699,44 @@ only use %d.\n", (count*2) + 2, FD_SETSIZE));
        FD_ZERO(pset);
 
        /* Add in the broadcast socket on 137. */
+       if (ClientNMB < 0 || ClientNMB >= FD_SETSIZE) {
+               errno = EBADF;
+               SAFE_FREE(pset);
+               return True;
+       }
+
        FD_SET(ClientNMB,pset);
        sock_array[num++] = ClientNMB;
        *maxfd = MAX( *maxfd, ClientNMB);
 
        /* Add in the 137 sockets on all the interfaces. */
        for (subrec = FIRST_SUBNET; subrec; subrec = NEXT_SUBNET_EXCLUDING_UNICAST(subrec)) {
+               if (subrec->nmb_sock < 0 || subrec->nmb_sock >= FD_SETSIZE) {
+                       /* We have to ignore sockets outside FD_SETSIZE. */
+                       continue;
+               }
                FD_SET(subrec->nmb_sock,pset);
                sock_array[num++] = subrec->nmb_sock;
                *maxfd = MAX( *maxfd, subrec->nmb_sock);
        }
 
        /* Add in the broadcast socket on 138. */
+       if (ClientDGRAM < 0 || ClientDGRAM >= FD_SETSIZE) {
+               errno = EBADF;
+               SAFE_FREE(pset);
+               return True;
+       }
+
        FD_SET(ClientDGRAM,pset);
        sock_array[num++] = ClientDGRAM;
        *maxfd = MAX( *maxfd, ClientDGRAM);
 
        /* Add in the 138 sockets on all the interfaces. */
        for (subrec = FIRST_SUBNET; subrec; subrec = NEXT_SUBNET_EXCLUDING_UNICAST(subrec)) {
+               if (subrec->dgram_sock < 0 || subrec->dgram_sock >= FD_SETSIZE) {
+                       /* We have to ignore sockets outside FD_SETSIZE. */
+                       continue;
+               }
                FD_SET(subrec->dgram_sock,pset);
                sock_array[num++] = subrec->dgram_sock;
                *maxfd = MAX( *maxfd, subrec->dgram_sock);
@@ -1767,7 +1787,7 @@ bool listen_for_packets(bool run_election)
 
 #ifndef SYNC_DNS
        dns_fd = asyncdns_fd();
-       if (dns_fd != -1) {
+       if (dns_fd >= 0 && dns_fd < FD_SETSIZE) {
                FD_SET(dns_fd, &r_fds);
                maxfd = MAX( maxfd, dns_fd);
        }

diff --git a/source/nsswitch/wb_common.c b/source/nsswitch/wb_common.c
index a1646215bdff3d44e7fe977f7cb3bdb2cc8c67f6..4f76bd0cffee9340cb543fb7e7e36db7fa346700 100644 (file)
--- a/source/nsswitch/wb_common.c
+++ b/source/nsswitch/wb_common.c
@@ -240,6 +240,12 @@ static int winbind_named_pipe_sock(const char *dir)
 
                switch (errno) {
                        case EINPROGRESS:
+
+                               if (fd < 0 || fd >= FD_SETSIZE) {
+                                       errno = EBADF;
+                                       goto error_out;
+                               }
+
                                FD_ZERO(&w_fds);
                                FD_SET(fd, &w_fds);
                                tv.tv_sec = CONNECT_TIMEOUT - wait_time;
@@ -383,7 +389,13 @@ int winbind_write_sock(void *buffer, int count, int recursing, int need_priv)
        while(nwritten < count) {
                struct timeval tv;
                fd_set r_fds;
-               
+
+               if (winbindd_fd < 0 || winbindd_fd >= FD_SETSIZE) {
+                       errno = EBADF;
+                       winbind_close_sock();
+                       return -1;
+               }
+
                /* Catch pipe close on other end by checking if a read()
                   call would not block by calling select(). */
 
@@ -443,7 +455,13 @@ int winbind_read_sock(void *buffer, int count)
        while(nread < count) {
                struct timeval tv;
                fd_set r_fds;
-               
+
+               if (winbindd_fd < 0 || winbindd_fd >= FD_SETSIZE) {
+                       errno = EBADF;
+                       winbind_close_sock();
+                       return -1;
+               }
+
                /* Catch pipe close on other end by checking if a read()
                   call would not block by calling select(). */
 

diff --git a/source/printing/printing.c b/source/printing/printing.c
index a9272ebcc23a2fddd0dbc34c241f47a688c72687..c3b8c6186d626f9145b5e70ad2e8ede651c5dad5 100644 (file)
--- a/source/printing/printing.c
+++ b/source/printing/printing.c
@@ -1407,6 +1407,11 @@ void start_background_queue(void)
                exit(1);
        }
 
+       if (pause_pipe[1] < 0 || pause_pipe[1] >= FD_SETSIZE) {
+               DEBUG(5,("start_background_queue: pipe fd out of range.\n"));
+               exit(1);
+       }
+
        background_lpq_updater_pid = sys_fork();
 
        if (background_lpq_updater_pid == -1) {

diff --git a/source/smbd/dnsregister.c b/source/smbd/dnsregister.c
index f02739ef8dfed67b1f8dcbb8e079e3ececce1a10..3c689b9cf3e9233088de523c25fb2bd57179877f 100644 (file)
--- a/source/smbd/dnsregister.c
+++ b/source/smbd/dnsregister.c
@@ -125,6 +125,9 @@ void dns_register_smbd(struct dns_reg_state ** dns_state_ptr,
         */
        if (dns_state->srv_ref != NULL) {
                mdnsd_conn_fd = DNSServiceRefSockFD(dns_state->srv_ref);
+               if (mdnsd_conn_fd < 0 || mdnsd_conn_fd >= FD_SETSIZE) {
+                       return;
+               }
                FD_SET(mdnsd_conn_fd, listen_set);
                return;
        }
@@ -156,6 +159,9 @@ void dns_register_smbd(struct dns_reg_state ** dns_state_ptr,
        }
 
        mdnsd_conn_fd = DNSServiceRefSockFD(dns_state->srv_ref);
+       if (mdnsd_conn_fd < 0 || mdnsd_conn_fd >= FD_SETSIZE) {
+               return;
+       }
        FD_SET(mdnsd_conn_fd, listen_set);
        *maxfd = MAX(*maxfd, mdnsd_conn_fd);
        *timeout = timeval_zero();

diff --git a/source/smbd/oplock.c b/source/smbd/oplock.c
index a07d05d080cb3f82f4dcae8e48d6556847a8f012..5ae3fdfe22e858c1c2509e03d7e9401ba36c3eab 100644 (file)
--- a/source/smbd/oplock.c
+++ b/source/smbd/oplock.c
@@ -241,7 +241,10 @@ bool downgrade_oplock(files_struct *fsp)
 int oplock_notify_fd(void)
 {
        if (koplocks) {
-               return koplocks->notification_fd;
+               int fd = koplocks->notification_fd;
+               if (fd < 0 || fd >= FD_SETSIZE) {
+                       return -1;
+               }
        }
 
        return -1;

diff --git a/source/smbd/oplock_irix.c b/source/smbd/oplock_irix.c
index 8c287c9836ebad8878f1b3694985952777afc2f8..6e86fac65b2dadc6aa82ebe43c9fa3d193e828e3 100644 (file)
--- a/source/smbd/oplock_irix.c
+++ b/source/smbd/oplock_irix.c
@@ -284,6 +284,11 @@ struct kernel_oplocks *irix_init_kernel_oplocks(void)
                return False;
        }
 
+       if (pfd[0] < 0 || pfd[0] >= FD_SETSIZE) {
+               DEBUG(0,("setup_kernel_oplock_pipe: fd out of range.\n"));
+               return False;
+       }
+
        oplock_pipe_read = pfd[0];
        oplock_pipe_write = pfd[1];
 

diff --git a/source/smbd/process.c b/source/smbd/process.c
index 403c7c65772e1301d1a9902114e3e39f53388c0c..9b8f29b771b792b1f7e798c3c0b4f3a66cc111b1 100644 (file)
--- a/source/smbd/process.c
+++ b/source/smbd/process.c
@@ -698,7 +698,7 @@ static void async_processing(fd_set *pfds)
 
 static int select_on_fd(int fd, int maxfd, fd_set *fds)
 {
-       if (fd != -1) {
+       if (fd != -1 && fd < FD_SETSIZE) {
                FD_SET(fd, fds);
                maxfd = MAX(maxfd, fd);
        }

diff --git a/source/smbd/server.c b/source/smbd/server.c
index 5129484730708bb569cf12c54370822f478f6ca8..a670334a106f5400f5614b38a5d7426d8010786b 100644 (file)
--- a/source/smbd/server.c
+++ b/source/smbd/server.c
@@ -209,7 +209,13 @@ static bool open_sockets_inetd(void)
        /* Started from inetd. fd 0 is the socket. */
        /* We will abort gracefully when the client or remote system 
           goes away */
-       smbd_set_server_fd(dup(0));
+       int fd = dup(0);
+
+       if (fd < 0 || fd >= FD_SETSIZE) {
+               return false;
+       }
+
+       smbd_set_server_fd(fd);
        
        /* close our standard file descriptors */
        close_low_fds(False); /* Don't close stderr */
@@ -436,7 +442,8 @@ static bool open_sockets_smbd(bool is_daemon, bool interactive, const char *smb_
                                                        num_sockets == 0 ? 0 : 2,
                                                        ifss,
                                                        true);
-                               if(s == -1) {
+                               if(s < 0 || s >= FD_SETSIZE) {
+                                       close(s);
                                        continue;
                                }
 
@@ -516,7 +523,7 @@ static bool open_sockets_smbd(bool is_daemon, bool interactive, const char *smb_
                                                num_sockets == 0 ? 0 : 2,
                                                &ss,
                                                true);
-                               if (s == -1) {
+                               if (s < 0 || s >= FD_SETSIZE) {
                                        continue;
                                }
 
@@ -709,6 +716,7 @@ static bool open_sockets_smbd(bool is_daemon, bool interactive, const char *smb_
                        struct sockaddr addr;
                        socklen_t in_addrlen = sizeof(addr);
                        pid_t child = 0;
+                       int fd;
 
                        s = -1;
                        for(i = 0; i < num_sockets; i++) {
@@ -721,16 +729,21 @@ static bool open_sockets_smbd(bool is_daemon, bool interactive, const char *smb_
                                }
                        }
 
-                       smbd_set_server_fd(accept(s,&addr,&in_addrlen));
-
-                       if (smbd_server_fd() == -1 && errno == EINTR)
+                       fd = accept(s,&addr,&in_addrlen);
+                       if (fd == -1 && errno == EINTR)
                                continue;
-
-                       if (smbd_server_fd() == -1) {
+                       if (fd == -1) {
                                DEBUG(2,("open_sockets_smbd: accept: %s\n",
                                         strerror(errno)));
                                continue;
                        }
+                       if (fd < 0 || fd >= FD_SETSIZE) {
+                               DEBUG(2,("open_sockets_smbd: bad fd %d\n",
+                                       fd ));
+                               continue;
+                       }
+
+                       smbd_set_server_fd(fd);
 
                        /* Ensure child is set to blocking mode */
                        set_blocking(smbd_server_fd(),True);

diff --git a/source/utils/smbfilter.c b/source/utils/smbfilter.c
index 1e22a402013491fa7e3851173b1059b93b1b1490..45f92079a5f867f90ba6e979b4ff78dc97cc65b7 100644 (file)
--- a/source/utils/smbfilter.c
+++ b/source/utils/smbfilter.c
@@ -162,8 +162,8 @@ static void filter_child(int c, struct sockaddr_storage *dest_ss)
                int num;
                
                FD_ZERO(&fds);
-               if (s != -1) FD_SET(s, &fds);
-               if (c != -1) FD_SET(c, &fds);
+               if (s >= 0 && s < FD_SETSIZE) FD_SET(s, &fds);
+               if (c >= 0 && c < FD_SETSIZE) FD_SET(c, &fds);
 
                num = sys_select_intr(MAX(s+1, c+1),&fds,NULL,NULL,NULL);
                if (num <= 0) continue;
@@ -235,6 +235,10 @@ static void start_filter(char *desthost)
                struct sockaddr_storage ss;
                socklen_t in_addrlen = sizeof(ss);
                
+               if (s < 0 || s >= FD_SETSIZE) {
+                       break;
+               }
+
                FD_ZERO(&fds);
                FD_SET(s, &fds);
 

diff --git a/source/winbindd/winbindd.c b/source/winbindd/winbindd.c
index 1d618e263174d4ea365b82c96e8eafea1e817a8f..6b5c251bf6b030b7bfd44bcd1dfd1c661281280f 100644 (file)
--- a/source/winbindd/winbindd.c
+++ b/source/winbindd/winbindd.c
@@ -836,7 +836,8 @@ static void process_loop(void)
        listen_sock = open_winbindd_socket();
        listen_priv_sock = open_winbindd_priv_socket();
 
-       if (listen_sock == -1 || listen_priv_sock == -1) {
+       if (listen_sock < 0 || listen_sock >= FD_SETSIZE ||
+                       listen_priv_sock < 0 || listen_priv_sock >= FD_SETSIZE) {
                perror("open_winbind_socket");
                exit(1);
        }
@@ -861,6 +862,9 @@ static void process_loop(void)
 
        FD_ZERO(&r_fds);
        FD_ZERO(&w_fds);
+
+       /* We check the range for listen_sock and
+          listen_priv_sock above. */
        FD_SET(listen_sock, &r_fds);
        FD_SET(listen_priv_sock, &r_fds);
 
@@ -890,6 +894,12 @@ static void process_loop(void)
        }
 
        for (ev = fd_events; ev; ev = ev->next) {
+               if (ev->fd < 0 || ev->fd >= FD_SETSIZE) {
+                       /* Ignore here - event_add_to_select_args
+                          should make this impossible. */
+                       continue;
+               }
+
                if (ev->flags & EVENT_FD_READ) {
                        FD_SET(ev->fd, &r_fds);
                        maxfd = MAX(ev->fd, maxfd);

diff --git a/source/winbindd/winbindd_dual.c b/source/winbindd/winbindd_dual.c
index ff004f293ced602ec965f1f7aa384eb2c98aabd6..b30ec20b4a1f703ff5d43c9ae6eaf29b7d8332d8 100644 (file)
--- a/source/winbindd/winbindd_dual.c
+++ b/source/winbindd/winbindd_dual.c
@@ -1250,6 +1250,12 @@ static bool fork_domain_child(struct winbindd_child *child)
                return False;
        }
 
+       if (fdpair[0] < 0 || fdpair[0] >= FD_SETSIZE) {
+               DEBUG(0, ("fork_domain_child: bad fd range (%d)\n", fdpair[0]));
+               errno = EBADF;
+               return False;
+       }
+
        ZERO_STRUCT(state);
        state.pid = sys_getpid();
 
@@ -1405,6 +1411,7 @@ static bool fork_domain_child(struct winbindd_child *child)
                message_dispatch(winbind_messaging_context());
 
                FD_ZERO(&read_fds);
+               /* We check state.sock against FD_SETSIZE above. */
                FD_SET(state.sock, &read_fds);
 
                ret = sys_select(state.sock + 1, &read_fds, NULL, NULL, tp);