• Kumar Kartikeya Dwivedi's avatar
    bpf: Do mark_chain_precision for ARG_CONST_ALLOC_SIZE_OR_ZERO · 2fc31465
    Kumar Kartikeya Dwivedi authored
    Precision markers need to be propagated whenever we have an ARG_CONST_*
    style argument, as the verifier cannot consider imprecise scalars to be
    equivalent for the purposes of states_equal check when such arguments
    refine the return value (in this case, set mem_size for PTR_TO_MEM). The
    resultant mem_size for the R0 is derived from the constant value, and if
    the verifier incorrectly prunes states considering them equivalent where
    such arguments exist (by seeing that both registers have reg->precise as
    false in regsafe), we can end up with invalid programs passing the
    verifier which can do access beyond what should have been the correct
    mem_size in that explored state.
    
    To show a concrete example of the problem:
    
    0000000000000000 <prog>:
           0:       r2 = *(u32 *)(r1 + 80)
           1:       r1 = *(u32 *)(r1 + 76)
           2:       r3 = r1
           3:       r3 += 4
           4:       if r3 > r2 goto +18 <LBB5_5>
           5:       w2 = 0
           6:       *(u32 *)(r1 + 0) = r2
           7:       r1 = *(u32 *)(r1 + 0)
           8:       r2 = 1
           9:       if w1 == 0 goto +1 <LBB5_3>
          10:       r2 = -1
    
    0000000000000058 <LBB5_3>:
          11:       r1 = 0 ll
          13:       r3 = 0
          14:       call bpf_ringbuf_reserve
          15:       if r0 == 0 goto +7 <LBB5_5>
          16:       r1 = r0
          17:       r1 += 16777215
          18:       w2 = 0
          19:       *(u8 *)(r1 + 0) = r2
          20:       r1 = r0
          21:       r2 = 0
          22:       call bpf_ringbuf_submit
    
    00000000000000b8 <LBB5_5>:
          23:       w0 = 0
          24:       exit
    
    For the first case, the single line execution's exploration will prune
    the search at insn 14 for the branch insn 9's second leg as it will be
    verified first using r2 = -1 (UINT_MAX), while as w1 at insn 9 will
    always be 0 so at runtime we don't get error for being greater than
    UINT_MAX/4 from bpf_ringbuf_reserve. The verifier during regsafe just
    sees reg->precise as false for both r2 registers in both states, hence
    considers them equal for purposes of states_equal.
    
    If we propagated precise markers using the backtracking support, we
    would use the precise marking to then ensure that old r2 (UINT_MAX) was
    within the new r2 (1) and this would never be true, so the verification
    would rightfully fail.
    
    The end result is that the out of bounds access at instruction 19 would
    be permitted without this fix.
    
    Note that reg->precise is always set to true when user does not have
    CAP_BPF (or when subprog count is greater than 1 (i.e. use of any static
    or global functions)), hence this is only a problem when precision marks
    need to be explicitly propagated (i.e. privileged users with CAP_BPF).
    
    A simplified test case has been included in the next patch to prevent
    future regressions.
    
    Fixes: 457f4436 ("bpf: Implement BPF ring buffer and verifier support for it")
    Signed-off-by: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
    Link: https://lore.kernel.org/r/20220823185300.406-2-memxor@gmail.comSigned-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
    2fc31465
verifier.c 437 KB