LLVM can generate code that tests for direct packet access via
skb->data/data_end in a way that currently gets rejected by the
verifier, example:

  [...]
   7: (61) r3 = *(u32 *)(r6 +80)
   8: (61) r9 = *(u32 *)(r6 +76)
   9: (bf) r2 = r9
  10: (07) r2 += 54
  11: (3d) if r3 >= r2 goto pc+12
   R1=inv R2=pkt(id=0,off=54,r=0) R3=pkt_end R4=inv R6=ctx
   R9=pkt(id=0,off=0,r=0) R10=fp
  12: (18) r4 = 0xffffff7a
  14: (05) goto pc+430
  [...]

  from 11 to 24: R1=inv R2=pkt(id=0,off=54,r=0) R3=pkt_end R4=inv
                 R6=ctx R9=pkt(id=0,off=0,r=0) R10=fp
  24: (7b) *(u64 *)(r10 -40) = r1
  25: (b7) r1 = 0
  26: (63) *(u32 *)(r6 +56) = r1
  27: (b7) r2 = 40
  28: (71) r8 = *(u8 *)(r9 +20)
  invalid access to packet, off=20 size=1, R9(id=0,off=0,r=0)

The reason why this gets rejected despite a proper test is that we
currently call find_good_pkt_pointers() only in case where we detect
tests like rX > pkt_end, where rX is of type pkt(id=Y,off=Z,r=0) and
derived, for example, from a register of type pkt(id=Y,off=0,r=0)
pointing to skb->data. find_good_pkt_pointers() then fills the range
in the current branch to pkt(id=Y,off=0,r=Z) on success.

For above case, we need to extend that to recognize pkt_end >= rX
pattern and mark the other branch that is taken on success with the
appropriate pkt(id=Y,off=0,r=Z) type via find_good_pkt_pointers().
Since eBPF operates on BPF_JGT (>) and BPF_JGE (>=), these are the
only two practical options to test for from what LLVM could have
generated, since there's no such thing as BPF_JLT (<) or BPF_JLE (<=)
that we would need to take into account as well.

After the fix:

  [...]
   7: (61) r3 = *(u32 *)(r6 +80)
   8: (61) r9 = *(u32 *)(r6 +76)
   9: (bf) r2 = r9
  10: (07) r2 += 54
  11: (3d) if r3 >= r2 goto pc+12
   R1=inv R2=pkt(id=0,off=54,r=0) R3=pkt_end R4=inv R6=ctx
   R9=pkt(id=0,off=0,r=0) R10=fp
  12: (18) r4 = 0xffffff7a
  14: (05) goto pc+430
  [...]

  from 11 to 24: R1=inv R2=pkt(id=0,off=54,r=54) R3=pkt_end R4=inv
                 R6=ctx R9=pkt(id=0,off=0,r=54) R10=fp
  24: (7b) *(u64 *)(r10 -40) = r1
  25: (b7) r1 = 0
  26: (63) *(u32 *)(r6 +56) = r1
  27: (b7) r2 = 40
  28: (71) r8 = *(u8 *)(r9 +20)
  29: (bf) r1 = r8
  30: (25) if r8 > 0x3c goto pc+47
   R1=inv56 R2=imm40 R3=pkt_end R4=inv R6=ctx R8=inv56
   R9=pkt(id=0,off=0,r=54) R10=fp
  31: (b7) r1 = 1
  [...]

Verifier test cases are also added in this work, one that demonstrates
the mentioned example here and one that tries a bad packet access for
the current/fall-through branch (the one with types pkt(id=X,off=Y,r=0),
pkt(id=X,off=0,r=0)), then a case with good and bad accesses, and two
with both test variants (>, >=).

Fixes: 969bf05e

 ("bpf: direct packet access")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>