On 9/28/22 7:04 PM, Eric Dumazet wrote:
On Fri, Sep 23, 2022 at 3:48 PM Martin KaFai Lau <kafai@xxxxxx> wrote:From: Martin KaFai Lau <martin.lau@xxxxxxxxxx> When a bad bpf prog '.init' calls bpf_setsockopt(TCP_CONGESTION, "itself"), it will trigger this loop: .init => bpf_setsockopt(tcp_cc) => .init => bpf_setsockopt(tcp_cc) ... ... => .init => bpf_setsockopt(tcp_cc). It was prevented by the prog->active counter before but the prog->active detection cannot be used in struct_ops as explained in the earlier patch of the set. In this patch, the second bpf_setsockopt(tcp_cc) is not allowed in order to break the loop. This is done by using a bit of an existing 1 byte hole in tcp_sock to check if there is on-going bpf_setsockopt(TCP_CONGESTION) in this tcp_sock. Note that this essentially limits only the first '.init' can call bpf_setsockopt(TCP_CONGESTION) to pick a fallback cc (eg. peer does not support ECN) and the second '.init' cannot fallback to another cc. This applies even the second bpf_setsockopt(TCP_CONGESTION) will not cause a loop. Signed-off-by: Martin KaFai Lau <martin.lau@xxxxxxxxxx> --- include/linux/tcp.h | 6 ++++++ net/core/filter.c | 28 +++++++++++++++++++++++++++- 2 files changed, 33 insertions(+), 1 deletion(-) diff --git a/include/linux/tcp.h b/include/linux/tcp.h index a9fbe22732c3..3bdf687e2fb3 100644 --- a/include/linux/tcp.h +++ b/include/linux/tcp.h @@ -388,6 +388,12 @@ struct tcp_sock { u8 bpf_sock_ops_cb_flags; /* Control calling BPF programs * values defined in uapi/linux/tcp.h */ + u8 bpf_chg_cc_inprogress:1; /* In the middle of + * bpf_setsockopt(TCP_CONGESTION), + * it is to avoid the bpf_tcp_cc->init() + * to recur itself by calling + * bpf_setsockopt(TCP_CONGESTION, "itself"). + */ #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) (TP->bpf_sock_ops_cb_flags & ARG) #else #define BPF_SOCK_OPS_TEST_FLAG(TP, ARG) 0 diff --git a/net/core/filter.c b/net/core/filter.c index 96f2f7a65e65..ac4c45c02da5 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -5105,6 +5105,9 @@ static int bpf_sol_tcp_setsockopt(struct sock *sk, int optname, static int sol_tcp_sockopt_congestion(struct sock *sk, char *optval, int *optlen, bool getopt) { + struct tcp_sock *tp; + int ret; + if (*optlen < 2) return -EINVAL; @@ -5125,8 +5128,31 @@ static int sol_tcp_sockopt_congestion(struct sock *sk, char *optval, if (*optlen >= sizeof("cdg") - 1 && !strncmp("cdg", optval, *optlen)) return -ENOTSUPP; - return do_tcp_setsockopt(sk, SOL_TCP, TCP_CONGESTION, + /* It stops this looping + * + * .init => bpf_setsockopt(tcp_cc) => .init => + * bpf_setsockopt(tcp_cc)" => .init => .... + * + * The second bpf_setsockopt(tcp_cc) is not allowed + * in order to break the loop when both .init + * are the same bpf prog. + * + * This applies even the second bpf_setsockopt(tcp_cc) + * does not cause a loop. This limits only the first + * '.init' can call bpf_setsockopt(TCP_CONGESTION) to + * pick a fallback cc (eg. peer does not support ECN) + * and the second '.init' cannot fallback to + * another. + */ + tp = tcp_sk(sk); + if (tp->bpf_chg_cc_inprogress) + return -EBUSY; +Is the socket locked (and owned by current thread) at this point ? If not, changing bpf_chg_cc_inprogress would be racy.
Yes, the socket is locked and owned. There is a sock_owned_by_me check earlier in _bpf_setsockopt().
+ tp->bpf_chg_cc_inprogress = 1; + ret = do_tcp_setsockopt(sk, SOL_TCP, TCP_CONGESTION, KERNEL_SOCKPTR(optval), *optlen); + tp->bpf_chg_cc_inprogress = 0; + return ret; } static int sol_tcp_sockopt(struct sock *sk, int optname, -- 2.30.2
