Hi all. I believe I spotted a race condition in virtio-net or qemu/kvm (but only when virtio-net is involved).
To replicate, one needs a virtualization environment similar to Host: - debian 12 x86_64, kernel 6.1.0-18-amd64 - caching name server listening on 127.0.0.1 - qemu version 7.2.9 (Debian 1:7.2+dfsg-7+deb12u5) - command line: qemu-system-x86_64 \ -enable-kvm \ -daemonize \ -parallel none \ -serial none \ -m 256 \ -drive if=virtio,format=raw,file=void.raw \ -monitor unix:run/void.mon,server,nowait \ -nic user,model=virtio,hostfwd=tcp:127.0.0.1:3822-:22 Guest: - x86_64, linux/musl or linux/glibc or freebsd or openbsd - /etc/resolv.conf: nameserver 10.0.2.2 i.e. the caching dns in the host nameserver 192.168.1.123 non existent and run the attached program in the guest. The program opens a UDP socket, sends out a bunch of (dns) requests, poll()s on the socket, and then receives the responses. If a delay is inserted between the sendto() calls, the (unique) response from the host is received correctly: $ ./a.out 10.0.2.2 >/dev/null # to warm up the host cache $ ./a.out 10.0.2.2 delay 192.168.1.123 poll: 1 1 1 recvfrom() 45 <response packet> recvfrom() -1 If the sento()s are performed in short order, the response packet gets lost: $ ./a.out 10.0.2.2 >/dev/null # to warm up the host cache $ ./a.out 10.0.2.2 192.168.1.123 poll: 0 1 0 recvfrom() -1 recvfrom() -1 A tcpdump capture on the host side shows no difference between the two cases. Tcpdump on the guest side is another story: in the good case, it looks like this 7:32:44.332 IP 10.0.2.15.43276 > 10.0.2.2.53: 33452+ A? example.com. (29) 7:32:44.333 IP 10.0.2.2.53 > 10.0.2.15.43276: 33452 1/0/0 A 93.184.216.34 (45) 7:32:44.349 IP 10.0.2.15.43276 > 192.168.1.123.53: 33452+ A? example.com. (29) while in the bad case it looks like this 7:32:55.358 IP 10.0.2.15.46537 > 10.0.2.2.53: 33452+ A? example.com. (29) 7:32:55.358 IP 10.0.2.15.46537 > 192.168.1.123.53: 33452+ A? example.com. (29) 7:32:55.358 IP *127.0.0.1*.53 > 10.0.2.15.46537: 33452 1/0/0 A 93.184.216.34 (45) where the response packet has wrong src ip. Looks like a failure of the NAT layer, but it does not happen when the guest uses another emulated network driver: don't know whether it's because the relevant code is in virtio-net or because other drivers add overhead that masks the issue. There's nothing special in port 53: I was just investigating a weird failure in name resolution in a MUSL based guest (https://www.openwall.com/lists/musl/2024/02/17/3) and wrote the program to mimic MUSL resolver's behaviour. But it succeeds/fails consistently with a different port, and in all guests I tried (as long as the emulated network device is virtio-net). To see the issue, it's important that the response to the first request is so fast that it's simultaneous with the second request: that's the reason behind the caching nameserver in the host. I also opened a bug report to debian (https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1064634). I'm not subscribed to qemu-devel, so please CC me in replies. Best regards, g.b.
#include <stdio.h> #include <time.h> #include <poll.h> #include <assert.h> #include <string.h> #include <arpa/inet.h> #include <netdb.h> #include <netinet/in.h> #include <sys/socket.h> #include <sys/socket.h> #include <sys/types.h> static void dump(const char *s, size_t len) { while (len--) { char t = *s++; if (' ' <= t && t <= '~' && t != '\\') printf("%c", t); else printf("\\%o", t & 0xff); } printf("\n"); } int main(int argc, char *argv[]) { int sock, rv, n; const char req[] = "\202\254\1\0\0\1\0\0\0\0\0\0\7example\3com\0\0\1\0\1"; struct timespec delay_l = { 1, 0 }; /* 1 sec */ struct pollfd pfs; struct sockaddr_in me = { 0 }; sock = socket(AF_INET, SOCK_DGRAM | SOCK_CLOEXEC | SOCK_NONBLOCK, IPPROTO_IP); assert(sock >= 0); me.sin_family = AF_INET; me.sin_port = 0; me.sin_addr.s_addr = inet_addr("0.0.0.0"); rv = bind(sock, (struct sockaddr *) &me, sizeof me); assert(0 == rv); for (n = 1; n < argc; n++) { if (0 == strcmp("delay", argv[n])) { struct timespec delay_s = { 0, (1 << 24) }; /* ~ 16 msec */ nanosleep(&delay_s, NULL); } else { struct sockaddr_in dst = { 0 }; dst.sin_family = AF_INET; dst.sin_port = htons(53); dst.sin_addr.s_addr = inet_addr(argv[n]); rv = sendto(sock, req, sizeof req - 1, MSG_NOSIGNAL, (struct sockaddr *) &dst, sizeof dst); assert(rv >= 0); } } nanosleep(&delay_l, NULL); pfs.fd = sock; pfs.events = POLLIN; rv = poll(&pfs, 1, 2000); printf("poll: %d %d %d\n", rv, pfs.events, pfs.revents); for (n = 1; n < argc; n++) { char resp[4000]; if (0 == strcmp("delay", argv[n])) continue; rv = recvfrom(sock, resp, sizeof resp, 0, NULL, NULL); printf("recvfrom() %d\n", rv); if (rv > 0) dump(resp, rv); } return 0; }