Additional info:
# gdb core.qemu-kvm.5.0.0-0.module+el8.3.0-info
(gdb) bt full
#0  0x00007eff1a5d17ff in raise () at /lib64/libc.so.6
#1  0x00007eff1a5bbc35 in abort () at /lib64/libc.so.6
#2  0x00007eff1a5bbb09 in _nl_load_domain.cold.0 () at /lib64/libc.so.6
#3  0x00007eff1a5c9de6 in .annobin_assert.c_end () at /lib64/libc.so.6
#4  0x0000560ecccb7f41 in qemu_set_nonblock (fd=fd@entry=9)
    at /usr/src/debug/qemu-kvm-5.0.0-0.module+el8.3.0+6620+5d5e1420.x86_64/util/oslib-posix.c:251
        f = <optimized out>
        __PRETTY_FUNCTION__ = "qemu_set_nonblock"
#5  0x0000560eccba1189 in net_init_tap
    (netdev=0x7ffda14b1530, name=0x560ece63ce30 "hostnet0", peer=0x0, errp=0x7ffda14b1528)
    at /usr/src/debug/qemu-kvm-5.0.0-0.module+el8.3.0+6620+5d5e1420.x86_64/net/tap.c:798
        tap = 0x7ffda14b1540
        fd = 9
--Type <RET> for more, q to quit, c to continue without paging--
        vnet_hdr = 0
        i = 0
        queues = 1
        script = 0x0
        downscript = 0x0
        err = 0x0
        vhostfdname = 0x0
        ifname = "X\000\000\000\000\000\000\000\001\000\000\000\000\000\000\000x\023K\241\375\177\000\000\377\377\377\377\000\000\000\000\000@\000\000\000\000\000\000\347ս\314\001\000\000\000\377\377\377\377\377\377\377\377\004\000\000\000\000\000\000\000\200\310[\316\016V\000\000@\366\016\341\374~\000\000\006\000\000\000\000\000\000\000\t)\t\037\377~\000\000\252\233\343\314\000\000\000\000\000\000\000\000\001\000\000\000\020\000\000\000\375\177\000\000\360t]\316\016V\000"
        __PRETTY_FUNCTION__ = "net_init_tap"
        __func__ = "net_init_tap"
#6  0x0000560eccb91078 in net_client_init1 (object=0x7ffda14b1530, is_netdev=<optimized out>, errp=0x7ffda14b1528)
    at /usr/src/debug/qemu-kvm-5.0.0-0.module+el8.3.0+6620+5d5e1420.x86_64/net/net.c:1055
        legacy = 
              {id = 0x0, type = NET_CLIENT_DRIVER_NONE, u = {nic = {has_netdev = false, netdev = 0x0, has_macaddr = false, macaddr = 0x0, has_model = false, model = 0x0, has_addr = false, addr = 0x0, has_vectors = false, vectors = 0}, user = {has_hostname = false, hostname = 0x0, has_q_restrict = false, q_restrict = false, has_ipv4 = false, ipv4 = false, has_ipv6 = false, ipv6 = false, has_ip = false, ip = 0x0, has_net = false, net = 0x0, has_host = false, host = 0x0, has_tftp = false, tftp = 0x0, has_bootfile = false, bootfile = 0x0, has_dhcpstart = false, dhcpstart = 0x0, has_dns = false, dns = 0x0, has_dnssearch = false, dnssearch = 0x0, has_domainname = false, domainname = 0x0, has_ipv6_prefix = false, ipv6_prefix = 0x0, has_ipv6_prefixlen = false, ipv6_prefixlen = 0, has_ipv6_host = false, ipv6_host = 0x0, has_ipv6_dns = false, ipv6_dns = 0x0, has_smb = false, smb = 0x0, has_smbserver = false, smbserver = 0x0, has_hostfwd = false, hostfwd = 0x0, has_guestfwd = false, guestfwd = 0x0, has_tftp_server_name = false, tftp_server_name = 0x0}, tap = {has_ifname = false, ifname = 0x0, has_fd = false, fd = 0x0, has_fds = false, fds = 0x0, has_script = false, script = 0x0, has_downscript = false, downscript = 0x0, has_br = false, br = 0x0, has_helper = false, helper = 0x0, has_sndbuf = false, sndbuf = 0, has_vnet_hdr = false, vnet_hdr = false, has_vhost = false, vhost = false, has_vhostfd = false, vhostfd = 0x0, has_vhostfds = false, vhostfds = 0x0, has_vhostforce = false, vhostforce = false, has_queues = false, queues = 0, has_poll_us = false, poll_us = 0}, l2tpv3 = {src = 0x0, dst = 0x0, has_srcport = false, srcport = 0x0, has--Type <RET> for more, q to quit, c to continue without paging--
_dstport = false, dstport = 0x0, has_ipv6 = false, ipv6 = false, has_udp = false, udp = false, has_cookie64 = false, cookie64 = false, has_counter = false, counter = false, has_pincounter = false, pincounter = false, has_txcookie = false, txcookie = 0, has_rxcookie = false, rxcookie = 0, txsession = 0, has_rxsession = false, rxsession = 0, has_offset = false, offset = 0}, socket = {has_fd = false, fd = 0x0, has_listen = false, listen = 0x0, has_connect = false, connect = 0x0, has_mcast = false, mcast = 0x0, has_localaddr = false, localaddr = 0x0, has_udp = false, udp = 0x0}, vde = {has_sock = false, sock = 0x0, has_port = false, port = 0, has_group = false, group = 0x0, has_mode = false, mode = 0}, bridge = {has_br = false, br = 0x0, has_helper = false, helper = 0x0}, hubport = {hubid = 0, has_netdev = false, netdev = 0x0}, netmap = {ifname = 0x0, has_devname = false, devname = 0x0}, vhost_user = {chardev = 0x0, has_vhostforce = false, vhostforce = false, has_queues = false, queues = 0}}}
        netdev = 0x7ffda14b1530
        name = <optimized out>
        peer = 0x0
        __func__ = "net_client_init1"
        __PRETTY_FUNCTION__ = "net_client_init1"
#7  0x0000560eccbaeec9 in qmp_marshal_netdev_add (args=<optimized out>, ret=<optimized out>, errp=0x7ffda14b16c8)
    at qapi/qapi-commands-net.c:76
        err = 0x0
        v = 0x560ecee2af20
        arg = 
              {id = 0x560ece63ce30 "hostnet0", type = NET_CLIENT_DRIVER_TAP, u = {nic = {has_netdev = false, netdev = 0x0, has_macaddr = true, macaddr = 0x560ececb7de0 "9", has_model = false, model = 0x0, has_addr = false, addr = 0x0, has_vectors = false, vectors = 0}, user = {has_hostname = false, hostname = 0x0, has_q_restrict = true, q_restrict = false, has_ipv4 = false, ipv4 = false, has_ipv6 = false, ipv6 = false, has_ip = false, ip = 0x560ececb7de0 "9", has_net = false, net = 0x0, has_host = false, host = 0x0, has_tftp = false, tftp = 0x0, has_bootfile = false, bootfile = 0x0, has_dhcpstart = false, dhcpstart = 0x0, has_dns = false, dns = 0x0, has_dnssearch = false, dnssearch = 0x0, has_domainname = false, domainname = 0x0, has_ipv6_prefix = false, ipv6_prefix = 0x0, has_ipv6_prefixlen = false, ipv6_prefixlen = 0, has_ipv6_host = false, ipv6_host = 0x0, has_ipv6_dns = false, ipv6_dns = 0x0, has_smb = false, smb = 0x0, has_smbserver = false, smbserver = 0x0, has_hostfwd = false, hostfwd = 0x0, has_guestfwd = false, guestfwd = 0x0, has_tftp_server_name = false, tftp_server_name = 0x0}, tap = {has_ifname = false, ifname = 0x0, has_fd = true, fd = 0x560ececb7de0 "9", has_fds = false, fds = 0x0, has_script = false, script = 0x0, has_downscript = false, downscript = 0x0, has_br = false, br = 0x0, has_helper = false, helper = 0x0, has_sndbuf = false, sndbuf = 0, has_vnet_hdr =--Type <RET> for more, q to quit, c to continue without paging--
 false, vnet_hdr = false, has_vhost = true, vhost = true, has_vhostfd = false, vhostfd = 0x0, has_vhostfds = false, vhostfds = 0x0, has_vhostforce = false, vhostforce = false, has_queues = false, queues = 0, has_poll_us = false, poll_us = 0}, l2tpv3 = {src = 0x0, dst = 0x0, has_srcport = true, srcport = 0x560ececb7de0 "9", has_dstport = false, dstport = 0x0, has_ipv6 = false, ipv6 = false, has_udp = false, udp = false, has_cookie64 = false, cookie64 = false, has_counter = false, counter = false, has_pincounter = false, pincounter = false, has_txcookie = false, txcookie = 0, has_rxcookie = false, rxcookie = 0, txsession = 0, has_rxsession = false, rxsession = 0, has_offset = false, offset = 0}, socket = {has_fd = false, fd = 0x0, has_listen = true, listen = 0x560ececb7de0 "9", has_connect = false, connect = 0x0, has_mcast = false, mcast = 0x0, has_localaddr = false, localaddr = 0x0, has_udp = false, udp = 0x0}, vde = {has_sock = false, sock = 0x0, has_port = true, port = 0, has_group = false, group = 0x560ececb7de0 "9", has_mode = false, mode = 0}, bridge = {has_br = false, br = 0x0, has_helper = true, helper = 0x560ececb7de0 "9"}, hubport = {hubid = 0, has_netdev = false, netdev = 0x0}, netmap = {ifname = 0x0, has_devname = false, devname = 0x1 <error: Cannot access memory at address 0x1>}, vhost_user = {chardev = 0x0, has_vhostforce = false, vhostforce = false, has_queues = false, queues = 1}}}
#8  0x0000560eccc69d70 in qmp_dispatch
    (cmds=0x560ecd4f7540 <qmp_commands>, request=<optimized out>, allow_oob=<optimized out>)

I think this happens because in tap_cleanup() the fd (9) is closed on netdev_del, and you can't reuse it with netdev_add.

It is another way to produce bug described in BZ 1708076

Perhaps in this case we might avoid to crash a running system.

This change should fix the problem:

diff --git a/net/tap.c b/net/tap.c
index ca48f2a285..f1bb1b99e8 100644
--- a/net/tap.c
+++ b/net/tap.c
@@ -795,6 +795,11 @@ int net_init_tap(const Netdev *netdev, const char *name,
             error_propagate(errp, err);
             return -1;
         }
+        /* Check if fd is valid */
+        if (fcntl(fd, F_GETFD) == -1) {
+            error_setg(errp, "Invalid file descriptor");
+            return -1;
+        }
 
         qemu_set_nonblock(fd);

Sent a new series upstream to fix the problem:

[PATCH v2 0/2] net: tap: check file descriptor can be used
https://patchew.org/QEMU/20200630145737.232095-1-lvivier@redhat.com/

Merged upstream in 5.1.0-rc1

e7b347d0bf64 net: detect errors from probing vnet hdr flag for TAP devices
             https://github.com/qemu/qemu/commit/e7b347d0bf640adb1c998d317eaf44d2d7cbd973
894022e61601 net: check if the file descriptor is valid before using it
             https://github.com/qemu/qemu/commit/894022e616016fe81745753f14adfbd680a1c7ee

*** Bug 1708076 has been marked as a duplicate of this bug. ***

===Verified with qemu-kvm-5.1.0-2.module+el8.3.0+7652+b30e6901.x86_64
1.Create a macvtap device.
# ip link add link switch name macvtap0 type macvtap mode bridge
# ip link set macvtap0 up
# ip -d link show macvtap0 
11: macvtap0@switch: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 500
    link/ether c2:8f:97:03:9c:1e brd ff:ff:ff:ff:ff:ff promiscuity 0 minmtu 68 maxmtu 65535 
    macvtap mode bridge addrgenmode eui64 numtxqueues 1 numrxqueues 1 gso_max_size 65536 gso_max_segs 65535 

2.Boot a rhel8.3 guest
qemu cli:
/usr/libexec/qemu-kvm \
-name 'avocado-vt-vm1'  \
-sandbox on  \
-machine q35 \
-device pcie-root-port,id=pcie-root-port-0,multifunction=on,bus=pcie.0,addr=0x1,chassis=1 \
-device pcie-pci-bridge,id=pcie-pci-bridge-0,addr=0x0,bus=pcie-root-port-0  \
-nodefaults \
-device VGA,bus=pcie.0,addr=0x2 \
-m 8G \
-smp 16,maxcpus=16,cores=8,threads=1,dies=1,sockets=2  \
-cpu 'Haswell-noTSX',+kvm_pv_unhalt \
-device pcie-root-port,id=pcie-root-port-1,port=0x1,addr=0x1.0x1,bus=pcie.0,chassis=2 \
-device qemu-xhci,id=usb1,bus=pcie-root-port-1,addr=0x0 \
-device usb-tablet,id=usb-tablet1,bus=usb1.0,port=1 \
-device pcie-root-port,id=pcie-root-port-2,port=0x2,addr=0x1.0x2,bus=pcie.0,chassis=3 \
-device virtio-scsi-pci,id=virtio_scsi_pci0,bus=pcie-root-port-2,addr=0x0 \
-blockdev node-name=file_image1,driver=file,aio=threads,filename=/home/rhel830-64-virtio-scsi.qcow2,cache.direct=on,cache.no-flush=off \
-blockdev node-name=drive_image1,driver=qcow2,cache.direct=on,cache.no-flush=off,file=file_image1 \
-device scsi-hd,id=image1,drive=drive_image1,write-cache=on \
-vnc :0  \
-rtc base=utc,clock=host,driftfix=slew  \
-boot menu=off,order=cdn,once=c,strict=off \
-enable-kvm \
-monitor stdio \
-device pcie-root-port,id=pcie_extra_root_port_0,multifunction=on,bus=pcie.0,addr=0x3,chassis=5 \
-qmp tcp:0:5555,server,nowait \
-device pcie-root-port,id=pcie-root-port-3,port=0x3,addr=0x1.0x3,bus=pcie.0,chassis=4 \
9<>/dev/tap11 \

2.Hot plug macvtap device,then hot unplug it.
{"execute":"qmp_capabilities"}
{"return": {}}
{"execute":"netdev_add","arguments":{"type":"tap","id":"hostnet0","vhost": true,"fd":"9"}}
{"return": {}}
{"execute":"device_add","arguments":{"driver":"virtio-net-pci","netdev":"hostnet0","mac":"c2:8f:97:03:9c:1e","id":"net0","bus":"pcie-root-port-3"}}
{"return": {}}
{"timestamp": {"seconds": 1598257197, "microseconds": 458232}, "event": "NIC_RX_FILTER_CHANGED", "data": {"name": "net0", "path": "/machine/peripheral/net0/virtio-backend"}}
{"execute":"device_del","arguments":{"id":"net0"}}
{"return": {}}
{"timestamp": {"seconds": 1598257282, "microseconds": 276488}, "event": "DEVICE_DELETED", "data": {"path": "/machine/peripheral/net0/virtio-backend"}}
{"timestamp": {"seconds": 1598257282, "microseconds": 332422}, "event": "DEVICE_DELETED", "data": {"device": "net0", "path": "/machine/peripheral/net0"}}
{"execute":"netdev_del","arguments":{"id":"hostnet0"}}
{"return": {}}

3.Hot plug this macvtap device again,qemu core dumped.
{"execute":"netdev_add","arguments":{"type":"tap","id":"hostnet0","vhost": true,"fd":"9"}}
{"error": {"class": "GenericError", "desc": "hostnet0: Can't use file descriptor 9: Bad file descriptor"}}

4.Guest does not core dump.

So this bug has been fixed very well. Move to 'VERIFIED'.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (virt:8.3 bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2020:5137