[PATCH bpf-next v2 0/3] bpf: Fix FIONREAD and copied_seq issues

Jiayuan Chen
2025-11-21T03:00:23Z

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

---

v1 -> v2: Use skmsg.sk instead of extending BPF_F_XXX macro
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 | 48 ++++-
net/core/skmsg.c | 29 ++-
net/ipv4/tcp_bpf.c | 26 ++-
net/ipv4/udp_bpf.c | 25 ++-
.../selftests/bpf/prog_tests/sockmap_basic.c | 203 +++++++++++++++++-
.../bpf/progs/test_sockmap_pass_prog.c | 8 +
6 files changed, 323 insertions(+), 16 deletions(-)

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2.43.0