• Daniel Borkmann's avatar
    bpf: mark dst unknown on inconsistent {s, u}bounds adjustments · 6f16101e
    Daniel Borkmann authored
    syzkaller generated a BPF proglet and triggered a warning with
    the following:
      0: (b7) r0 = 0
      1: (d5) if r0 s<= 0x0 goto pc+0
       R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
      2: (1f) r0 -= r1
       R0=inv0 R1=ctx(id=0,off=0,imm=0) R10=fp0
      verifier internal error: known but bad sbounds
    What happens is that in the first insn, r0's min/max value
    are both 0 due to the immediate assignment, later in the jsle
    test the bounds are updated for the min value in the false
    path, meaning, they yield smin_val = 1, smax_val = 0, and when
    ctx pointer is subtracted from r0, verifier bails out with the
    internal error and throwing a WARN since smin_val != smax_val
    for the known constant.
    For min_val > max_val scenario it means that reg_set_min_max()
    and reg_set_min_max_inv() (which both refine existing bounds)
    demonstrated that such branch cannot be taken at runtime.
    In above scenario for the case where it will be taken, the
    existing [0, 0] bounds are kept intact. Meaning, the rejection
    is not due to a verifier internal error, and therefore the
    WARN() is not necessary either.
    We could just reject such cases in adjust_{ptr,scalar}_min_max_vals()
    when either known scalars have smin_val != smax_val or
    umin_val != umax_val or any scalar reg with bounds
    smin_val > smax_val or umin_val > umax_val. However, there
    may be a small risk of breakage of buggy programs, so handle
    this more gracefully and in adjust_{ptr,scalar}_min_max_vals()
    just taint the dst reg as unknown scalar when we see ops with
    such kind of src reg.
    Reported-by: syzbot+6d362cadd45dc0a12ba4@syzkaller.appspotmail.com
    Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
    Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
verifier.c 139 KB