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    Subject[PATCH 5.18 050/181] bpf: Fix request_sock leak in sk lookup helpers
    Date
    From: Jon Maxwell <jmaxwell37@gmail.com>

    [ Upstream commit 3046a827316c0e55fc563b4fb78c93b9ca5c7c37 ]

    A customer reported a request_socket leak in a Calico cloud environment. We
    found that a BPF program was doing a socket lookup with takes a refcnt on
    the socket and that it was finding the request_socket but returning the parent
    LISTEN socket via sk_to_full_sk() without decrementing the child request socket
    1st, resulting in request_sock slab object leak. This patch retains the
    existing behaviour of returning full socks to the caller but it also decrements
    the child request_socket if one is present before doing so to prevent the leak.

    Thanks to Curtis Taylor for all the help in diagnosing and testing this. And
    thanks to Antoine Tenart for the reproducer and patch input.

    v2 of this patch contains, refactor as per Daniel Borkmann's suggestions to
    validate RCU flags on the listen socket so that it balances with bpf_sk_release()
    and update comments as per Martin KaFai Lau's suggestion. One small change to
    Daniels suggestion, put "sk = sk2" under "if (sk2 != sk)" to avoid an extra
    instruction.

    Fixes: f7355a6c0497 ("bpf: Check sk_fullsock() before returning from bpf_sk_lookup()")
    Fixes: edbf8c01de5a ("bpf: add skc_lookup_tcp helper")
    Co-developed-by: Antoine Tenart <atenart@kernel.org>
    Signed-off-by: Antoine Tenart <atenart@kernel.org>
    Signed-off-by: Jon Maxwell <jmaxwell37@gmail.com>
    Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
    Tested-by: Curtis Taylor <cutaylor-pub@yahoo.com>
    Cc: Martin KaFai Lau <kafai@fb.com>
    Link: https://lore.kernel.org/bpf/56d6f898-bde0-bb25-3427-12a330b29fb8@iogearbox.net
    Link: https://lore.kernel.org/bpf/20220615011540.813025-1-jmaxwell37@gmail.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    net/core/filter.c | 34 ++++++++++++++++++++++++++++------
    1 file changed, 28 insertions(+), 6 deletions(-)

    diff --git a/net/core/filter.c b/net/core/filter.c
    index 8847316ee20e..af1e77f2f24a 100644
    --- a/net/core/filter.c
    +++ b/net/core/filter.c
    @@ -6506,10 +6506,21 @@ __bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
    ifindex, proto, netns_id, flags);

    if (sk) {
    - sk = sk_to_full_sk(sk);
    - if (!sk_fullsock(sk)) {
    + struct sock *sk2 = sk_to_full_sk(sk);
    +
    + /* sk_to_full_sk() may return (sk)->rsk_listener, so make sure the original sk
    + * sock refcnt is decremented to prevent a request_sock leak.
    + */
    + if (!sk_fullsock(sk2))
    + sk2 = NULL;
    + if (sk2 != sk) {
    sock_gen_put(sk);
    - return NULL;
    + /* Ensure there is no need to bump sk2 refcnt */
    + if (unlikely(sk2 && !sock_flag(sk2, SOCK_RCU_FREE))) {
    + WARN_ONCE(1, "Found non-RCU, unreferenced socket!");
    + return NULL;
    + }
    + sk = sk2;
    }
    }

    @@ -6543,10 +6554,21 @@ bpf_sk_lookup(struct sk_buff *skb, struct bpf_sock_tuple *tuple, u32 len,
    flags);

    if (sk) {
    - sk = sk_to_full_sk(sk);
    - if (!sk_fullsock(sk)) {
    + struct sock *sk2 = sk_to_full_sk(sk);
    +
    + /* sk_to_full_sk() may return (sk)->rsk_listener, so make sure the original sk
    + * sock refcnt is decremented to prevent a request_sock leak.
    + */
    + if (!sk_fullsock(sk2))
    + sk2 = NULL;
    + if (sk2 != sk) {
    sock_gen_put(sk);
    - return NULL;
    + /* Ensure there is no need to bump sk2 refcnt */
    + if (unlikely(sk2 && !sock_flag(sk2, SOCK_RCU_FREE))) {
    + WARN_ONCE(1, "Found non-RCU, unreferenced socket!");
    + return NULL;
    + }
    + sk = sk2;
    }
    }

    --
    2.35.1


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