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    Subject[PATCH 4.14 017/159] sctp: set frag_point in sctp_setsockopt_maxseg correctly
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Xin Long <lucien.xin@gmail.com>

    commit ecca8f88da5c4260cc2bccfefd2a24976704c366 upstream.

    Now in sctp_setsockopt_maxseg user_frag or frag_point can be set with
    val >= 8 and val <= SCTP_MAX_CHUNK_LEN. But both checks are incorrect.

    val >= 8 means frag_point can even be less than SCTP_DEFAULT_MINSEGMENT.
    Then in sctp_datamsg_from_user(), when it's value is greater than cookie
    echo len and trying to bundle with cookie echo chunk, the first_len will
    overflow.

    The worse case is when it's value is equal as cookie echo len, first_len
    becomes 0, it will go into a dead loop for fragment later on. In Hangbin
    syzkaller testing env, oom was even triggered due to consecutive memory
    allocation in that loop.

    Besides, SCTP_MAX_CHUNK_LEN is the max size of the whole chunk, it should
    deduct the data header for frag_point or user_frag check.

    This patch does a proper check with SCTP_DEFAULT_MINSEGMENT subtracting
    the sctphdr and datahdr, SCTP_MAX_CHUNK_LEN subtracting datahdr when
    setting frag_point via sockopt. It also improves sctp_setsockopt_maxseg
    codes.

    Suggested-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
    Reported-by: Hangbin Liu <liuhangbin@gmail.com>
    Signed-off-by: Xin Long <lucien.xin@gmail.com>
    Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    include/net/sctp/sctp.h | 3 ++-
    net/sctp/socket.c | 29 +++++++++++++++++++----------
    2 files changed, 21 insertions(+), 11 deletions(-)

    --- a/include/net/sctp/sctp.h
    +++ b/include/net/sctp/sctp.h
    @@ -444,7 +444,8 @@ static inline int sctp_frag_point(const
    if (asoc->user_frag)
    frag = min_t(int, frag, asoc->user_frag);

    - frag = SCTP_TRUNC4(min_t(int, frag, SCTP_MAX_CHUNK_LEN));
    + frag = SCTP_TRUNC4(min_t(int, frag, SCTP_MAX_CHUNK_LEN -
    + sizeof(struct sctp_data_chunk)));

    return frag;
    }
    --- a/net/sctp/socket.c
    +++ b/net/sctp/socket.c
    @@ -3136,9 +3136,9 @@ static int sctp_setsockopt_mappedv4(stru
    */
    static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned int optlen)
    {
    + struct sctp_sock *sp = sctp_sk(sk);
    struct sctp_assoc_value params;
    struct sctp_association *asoc;
    - struct sctp_sock *sp = sctp_sk(sk);
    int val;

    if (optlen == sizeof(int)) {
    @@ -3154,26 +3154,35 @@ static int sctp_setsockopt_maxseg(struct
    if (copy_from_user(&params, optval, optlen))
    return -EFAULT;
    val = params.assoc_value;
    - } else
    + } else {
    return -EINVAL;
    + }

    - if ((val != 0) && ((val < 8) || (val > SCTP_MAX_CHUNK_LEN)))
    - return -EINVAL;
    + if (val) {
    + int min_len, max_len;

    - asoc = sctp_id2assoc(sk, params.assoc_id);
    - if (!asoc && params.assoc_id && sctp_style(sk, UDP))
    - return -EINVAL;
    + min_len = SCTP_DEFAULT_MINSEGMENT - sp->pf->af->net_header_len;
    + min_len -= sizeof(struct sctphdr) +
    + sizeof(struct sctp_data_chunk);
    +
    + max_len = SCTP_MAX_CHUNK_LEN - sizeof(struct sctp_data_chunk);

    + if (val < min_len || val > max_len)
    + return -EINVAL;
    + }
    +
    + asoc = sctp_id2assoc(sk, params.assoc_id);
    if (asoc) {
    if (val == 0) {
    - val = asoc->pathmtu;
    - val -= sp->pf->af->net_header_len;
    + val = asoc->pathmtu - sp->pf->af->net_header_len;
    val -= sizeof(struct sctphdr) +
    - sizeof(struct sctp_data_chunk);
    + sizeof(struct sctp_data_chunk);
    }
    asoc->user_frag = val;
    asoc->frag_point = sctp_frag_point(asoc, asoc->pathmtu);
    } else {
    + if (params.assoc_id && sctp_style(sk, UDP))
    + return -EINVAL;
    sp->user_frag = val;
    }


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