1. 08 6月, 2018 1 次提交
    • D
      bpf: reject passing modified ctx to helper functions · 58990d1f
      Daniel Borkmann 提交于
      As commit 28e33f9d ("bpf: disallow arithmetic operations on
      context pointer") already describes, f1174f77 ("bpf/verifier:
      rework value tracking") removed the specific white-listed cases
      we had previously where we would allow for pointer arithmetic in
      order to further generalize it, and allow e.g. context access via
      modified registers. While the dereferencing of modified context
      pointers had been forbidden through 28e33f9d, syzkaller did
      recently manage to trigger several KASAN splats for slab out of
      bounds access and use after frees by simply passing a modified
      context pointer to a helper function which would then do the bad
      access since verifier allowed it in adjust_ptr_min_max_vals().
      
      Rejecting arithmetic on ctx pointer in adjust_ptr_min_max_vals()
      generally could break existing programs as there's a valid use
      case in tracing in combination with passing the ctx to helpers as
      bpf_probe_read(), where the register then becomes unknown at
      verification time due to adding a non-constant offset to it. An
      access sequence may look like the following:
      
        offset = args->filename;  /* field __data_loc filename */
        bpf_probe_read(&dst, len, (char *)args + offset); // args is ctx
      
      There are two options: i) we could special case the ctx and as
      soon as we add a constant or bounded offset to it (hence ctx type
      wouldn't change) we could turn the ctx into an unknown scalar, or
      ii) we generalize the sanity test for ctx member access into a
      small helper and assert it on the ctx register that was passed
      as a function argument. Fwiw, latter is more obvious and less
      complex at the same time, and one case that may potentially be
      legitimate in future for ctx member access at least would be for
      ctx to carry a const offset. Therefore, fix follows approach
      from ii) and adds test cases to BPF kselftests.
      
      Fixes: f1174f77 ("bpf/verifier: rework value tracking")
      Reported-by: syzbot+3d0b2441dbb71751615e@syzkaller.appspotmail.com
      Reported-by: syzbot+c8504affd4fdd0c1b626@syzkaller.appspotmail.com
      Reported-by: syzbot+e5190cb881d8660fb1a3@syzkaller.appspotmail.com
      Reported-by: syzbot+efae31b384d5badbd620@syzkaller.appspotmail.com
      Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
      Acked-by: NAlexei Starovoitov <ast@kernel.org>
      Acked-by: NYonghong Song <yhs@fb.com>
      Acked-by: NEdward Cree <ecree@solarflare.com>
      Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
      58990d1f
  2. 05 6月, 2018 1 次提交
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