syzkaller reported a bug [1] where a socket using sockmap, after being unloaded, exposed incorrect copied_seq calculation. The selftest I provided can be used to reproduce the issue reported by syzkaller.
TCP recvmsg seq # bug 2: copied E92C873, seq E68D125, rcvnxt E7CEB7C, fl 40 WARNING: CPU: 1 PID: 5997 at net/ipv4/tcp.c:2724 tcp_recvmsg_locked+0xb2f/0x2910 net/ipv4/tcp.c:2724 Call Trace: <TASK> receive_fallback_to_copy net/ipv4/tcp.c:1968 [inline] tcp_zerocopy_receive+0x131a/0x2120 net/ipv4/tcp.c:2200 do_tcp_getsockopt+0xe28/0x26c0 net/ipv4/tcp.c:4713 tcp_getsockopt+0xdf/0x100 net/ipv4/tcp.c:4812 do_sock_getsockopt+0x34d/0x440 net/socket.c:2421 __sys_getsockopt+0x12f/0x260 net/socket.c:2450 __do_sys_getsockopt net/socket.c:2457 [inline] __se_sys_getsockopt net/socket.c:2454 [inline] __x64_sys_getsockopt+0xbd/0x160 net/socket.c:2454 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xcd/0xfa0 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f
A sockmap socket maintains its own receive queue (ingress_msg) which may contain data from either its own protocol stack or forwarded from other sockets.
FD1:read() -- FD1->copied_seq++ | [read data] | [enqueue data] v [sockmap] -> ingress to self -> ingress_msg queue FD1 native stack ------> ^ -- FD1->rcv_nxt++ -> redirect to other | [enqueue data] | | | ingress to FD1 v ^ ... | [sockmap] FD2 native stack
The issue occurs when reading from ingress_msg: we update tp->copied_seq by default, but if the data comes from other sockets (not the socket's own protocol stack), tcp->rcv_nxt remains unchanged. Later, when converting back to a native socket, reads may fail as copied_seq could be significantly larger than rcv_nxt.
Additionally, FIONREAD calculation based on copied_seq and rcv_nxt is insufficient for sockmap sockets, requiring separate field tracking.
[1] https://syzkaller.appspot.com/bug?extid=06dbd397158ec0ea4983
--- v5 -> v6: Some modifications suggested by Jakub Sitnicki, and added Reviewed-by tag. https://lore.kernel.org/bpf/20260106051458.279151-1-jiayuan.chen@linux.dev/
v1 -> v5: Use skmsg.sk instead of extending BPF_F_XXX macro and fix CI failure reported by CI v1: https://lore.kernel.org/bpf/20251117110736.293040-1-jiayuan.chen@linux.dev/
Jiayuan Chen (3): bpf, sockmap: Fix incorrect copied_seq calculation bpf, sockmap: Fix FIONREAD for sockmap bpf, selftest: Add tests for FIONREAD and copied_seq
include/linux/skmsg.h | 70 +++++- net/core/skmsg.c | 28 ++- net/ipv4/tcp_bpf.c | 26 ++- net/ipv4/udp_bpf.c | 20 +- .../selftests/bpf/prog_tests/sockmap_basic.c | 205 +++++++++++++++++- .../bpf/progs/test_sockmap_pass_prog.c | 14 ++ 6 files changed, 346 insertions(+), 17 deletions(-)