• Alexei Starovoitov's avatar
    bpf: prevent out-of-bounds speculation · b2157399
    Alexei Starovoitov authored
    Under speculation, CPUs may mis-predict branches in bounds checks. Thus,
    memory accesses under a bounds check may be speculated even if the
    bounds check fails, providing a primitive for building a side channel.
    
    To avoid leaking kernel data round up array-based maps and mask the index
    after bounds check, so speculated load with out of bounds index will load
    either valid value from the array or zero from the padded area.
    
    Unconditionally mask index for all array types even when max_entries
    are not rounded to power of 2 for root user.
    When map is created by unpriv user generate a sequence of bpf insns
    that includes AND operation to make sure that JITed code includes
    the same 'index & index_mask' operation.
    
    If prog_array map is created by unpriv user replace
      bpf_tail_call(ctx, map, index);
    with
      if (index >= max_entries) {
        index &= map->index_mask;
        bpf_tail_call(ctx, map, index);
      }
    (along with roundup to power 2) to prevent out-of-bounds speculation.
    There is secondary redundant 'if (index >= max_entries)' in the interpreter
    and in all JITs, but they can be optimized later if necessary.
    
    Other array-like maps (cpumap, devmap, sockmap, perf_event_array, cgroup_array)
    cannot be used by unpriv, so no changes there.
    
    That fixes bpf side of "Variant 1: bounds check bypass (CVE-2017-5753)" on
    all architectures with and without JIT.
    
    v2->v3:
    Daniel noticed that attack potentially can be crafted via syscall commands
    without loading the program, so add masking to those paths as well.
    Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
    Acked-by: default avatarJohn Fastabend <john.fastabend@gmail.com>
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    b2157399
arraymap.c 17.5 KB