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Message-ID: <591A23E3.2050105@iogearbox.net> Date: Mon, 15 May 2017 23:55:47 +0200 From: Daniel Borkmann <daniel@...earbox.net> To: David Miller <davem@...emloft.net> CC: ast@...com, alexei.starovoitov@...il.com, netdev@...r.kernel.org Subject: Re: [PATCH 2/3] bpf: Track alignment of MAP pointers in verifier. On 05/15/2017 05:34 PM, David Miller wrote: > From: Daniel Borkmann <daniel@...earbox.net> > Date: Mon, 15 May 2017 15:10:02 +0200 > >>>> What are the semantics of using id here? In ptr_to_pkt, we have it, >>>> so that eventually, in find_good_pkt_pointers() we can match on id >>>> and update the range for all such regs with the same id. I'm just >>>> wondering as the side effect of this is that this makes state >>>> pruning worse. > > Daniel, I looked at the state pruning for maps. The situation is > quite interesting. > > Once we have env->varlen_map_value_access (and load or store via a > PTR_TO_MAP_VALUE_ADJ pointer), the state pruning gets more strict, the > relevant tests are: > > if (memcmp(rold, rcur, sizeof(*rold)) == 0) > continue; > > /* If the ranges were not the same, but everything else was and > * we didn't do a variable access into a map then we are a-ok. > */ > if (!varlen_map_access && > memcmp(rold, rcur, offsetofend(struct bpf_reg_state, id)) == 0) > continue; > > The first memcmp() is not going to match any time we adjust any > component of a MAP pointer reg. The offset, the alignment, anything. > That means any side effect whatsoever performed by check_pointer_add() > even if we changed the code to not modify reg->id > > The second check elides: > > s64 min_value; > u64 max_value; > u32 min_align; > u32 aux_off; > u32 aux_off_align; > > from the comparison but only if we haven't done a variable length > MAP access. I'm actually wondering about the min_align/aux_off/aux_off_align and given this is not really related to varlen_map_access and we currently just skip this. We should make sure that when env->strict_alignment is false that we ignore any difference in min_align/aux_off/aux_off_align, afaik, the min_align would also be set on regs other than ptr_to_pkt. What about compare_ptrs_to_packet() for when env->strict_alignment is true in ptr_to_pkt case? Could we have a situation that prunes the search with matching the third test? Say, in the old case, we did go all the way and ... R3(off=0, r=0) R4 = R3 + 20 // AAA // now R4(off=20,r=0) if (R4 > data_end) got out; // BBB: now R4(off=20,r=20), R3(off=0,r=20) and R3 used to access ... verify the code under 'BBB' and found that it's safe to run, including alignment, etc. Next time we come to this branch through R4 = R3 + 33 (under AAA), so we have R4(off=33,r=0). What happens if we then do R4-=4, and access 4 bytes of the packet? The old R4(off=20,r=20) becomes R4(off=16,r=20), which was found safe and the new R4(off=33,r=0) becomes R4(off=29,r=33) which would end up being unaligned? Looks like we shouldn't prune in such case? Maybe test_verifier test case helps to visualize. > The only conclusion I can come to is that changing reg->id for > PTR_TO_MAP_VALUE_ADJ has no side effect for state pruning, unless we > perform PTR_TO_MAP_VALUE_ADJ register adjustments without ever > accessing the map via that pointer in the entire program. Why entire program, just between two state pruning points, no? (They are marked as STATE_LIST_MARK.) > I could add some new state to avoid the reg->id change, but given > the above I don't think that it is really necessary.
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