• Shmulik Ladkani's avatar
    net: gso: Fix skb_segment splat when splitting gso_size mangled skb having linear-headed frag_list · 3dcbdb13
    Shmulik Ladkani authored
    Historically, support for frag_list packets entering skb_segment() was
    limited to frag_list members terminating on exact same gso_size
    boundaries. This is verified with a BUG_ON since commit 89319d38
    ("net: Add frag_list support to skb_segment"), quote:
        As such we require all frag_list members terminate on exact MSS
        boundaries.  This is checked using BUG_ON.
        As there should only be one producer in the kernel of such packets,
        namely GRO, this requirement should not be difficult to maintain.
    However, since commit 6578171a ("bpf: add bpf_skb_change_proto helper"),
    the "exact MSS boundaries" assumption no longer holds:
    An eBPF program using bpf_skb_change_proto() DOES modify 'gso_size', but
    leaves the frag_list members as originally merged by GRO with the
    original 'gso_size'. Example of such programs are bpf-based NAT46 or
    This lead to a kernel BUG_ON for flows involving:
     - GRO generating a frag_list skb
     - bpf program performing bpf_skb_change_proto() or bpf_skb_adjust_room()
     - skb_segment() of the skb
    See example BUG_ON reports in [0].
    In commit 13acc94e ("net: permit skb_segment on head_frag frag_list skb"),
    skb_segment() was modified to support the "gso_size mangling" case of
    a frag_list GRO'ed skb, but *only* for frag_list members having
    head_frag==true (having a page-fragment head).
    Alas, GRO packets having frag_list members with a linear kmalloced head
    (head_frag==false) still hit the BUG_ON.
    This commit adds support to skb_segment() for a 'head_skb' packet having
    a frag_list whose members are *non* head_frag, with gso_size mangled, by
    disabling SG and thus falling-back to copying the data from the given
    'head_skb' into the generated segmented skbs - as suggested by Willem de
    Bruijn [1].
    Since this approach involves the penalty of skb_copy_and_csum_bits()
    when building the segments, care was taken in order to enable this
    solution only when required:
     - untrusted gso_size, by testing SKB_GSO_DODGY is set
       (SKB_GSO_DODGY is set by any gso_size mangling functions in
     - the frag_list is non empty, its item is a non head_frag, *and* the
       headlen of the given 'head_skb' does not match the gso_size.
    Fixes: 6578171a
     ("bpf: add bpf_skb_change_proto helper")
    Suggested-by: default avatarWillem de Bruijn <willemdebruijn.kernel@gmail.com>
    Cc: Daniel Borkmann <daniel@iogearbox.net>
    Cc: Eric Dumazet <eric.dumazet@gmail.com>
    Cc: Alexander Duyck <alexander.duyck@gmail.com>
    Signed-off-by: default avatarShmulik Ladkani <shmulik.ladkani@gmail.com>
    Reviewed-by: default avatarWillem de Bruijn <willemb@google.com>
    Reviewed-by: default avatarAlexander Duyck <alexander.h.duyck@linux.intel.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>