linux.git/kernel/bpf, branch v4.12

bpf: prevent leaking pointer via xadd on unpriviledged

2017-06-29T19:44:34+00:00

Leaking kernel addresses on unpriviledged is generally disallowed,
for example, verifier rejects the following:

  0: (b7) r0 = 0
  1: (18) r2 = 0xffff897e82304400
  3: (7b) *(u64 *)(r1 +48) = r2
  R2 leaks addr into ctx

Doing pointer arithmetic on them is also forbidden, so that they
don't turn into unknown value and then get leaked out. However,
there's xadd as a special case, where we don't check the src reg
for being a pointer register, e.g. the following will pass:

  0: (b7) r0 = 0
  1: (7b) *(u64 *)(r1 +48) = r0
  2: (18) r2 = 0xffff897e82304400 ; map
  4: (db) lock *(u64 *)(r1 +48) += r2
  5: (95) exit

We could store the pointer into skb->cb, loose the type context,
and then read it out from there again to leak it eventually out
of a map value. Or more easily in a different variant, too:

   0: (bf) r6 = r1
   1: (7a) *(u64 *)(r10 -8) = 0
   2: (bf) r2 = r10
   3: (07) r2 += -8
   4: (18) r1 = 0x0
   6: (85) call bpf_map_lookup_elem#1
   7: (15) if r0 == 0x0 goto pc+3
   R0=map_value(ks=8,vs=8,id=0),min_value=0,max_value=0 R6=ctx R10=fp
   8: (b7) r3 = 0
   9: (7b) *(u64 *)(r0 +0) = r3
  10: (db) lock *(u64 *)(r0 +0) += r6
  11: (b7) r0 = 0
  12: (95) exit

  from 7 to 11: R0=inv,min_value=0,max_value=0 R6=ctx R10=fp
  11: (b7) r0 = 0
  12: (95) exit

Prevent this by checking xadd src reg for pointer types. Also
add a couple of test cases related to this.

Fixes: 1be7f75d1668 ("bpf: enable non-root eBPF programs")
Fixes: 17a5267067f3 ("bpf: verifier (add verifier core)")
Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Acked-by: Martin KaFai Lau 
Acked-by: Edward Cree 
Signed-off-by: David S. Miller

bpf: fix wrong exposure of map_flags into fdinfo for lpm

2017-05-25T17:44:28+00:00

trie_alloc() always needs to have BPF_F_NO_PREALLOC passed in via
attr->map_flags, since it does not support preallocation yet. We
check the flag, but we never copy the flag into trie->map.map_flags,
which is later on exposed into fdinfo and used by loaders such as
iproute2. Latter uses this in bpf_map_selfcheck_pinned() to test
whether a pinned map has the same spec as the one from the BPF obj
file and if not, bails out, which is currently the case for lpm
since it exposes always 0 as flags.

Also copy over flags in array_map_alloc() and stack_map_alloc().
They always have to be 0 right now, but we should make sure to not
miss to copy them over at a later point in time when we add actual
flags for them to use.

Fixes: b95a5c4db09b ("bpf: add a longest prefix match trie map implementation")
Reported-by: Jarno Rajahalme 
Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: David S. Miller

bpf: properly reset caller saved regs after helper call and ld_abs/ind

2017-05-25T17:44:27+00:00

Currently, after performing helper calls, we clear all caller saved
registers, that is r0 - r5 and fill r0 depending on struct bpf_func_proto
specification. The way we reset these regs can affect pruning decisions
in later paths, since we only reset register's imm to 0 and type to
NOT_INIT. However, we leave out clearing of other variables such as id,
min_value, max_value, etc, which can later on lead to pruning mismatches
due to stale data.

Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: David S. Miller

bpf: fix incorrect pruning decision when alignment must be tracked

2017-05-25T17:44:27+00:00

Currently, when we enforce alignment tracking on direct packet access,
the verifier lets the following program pass despite doing a packet
write with unaligned access:

  0: (61) r2 = *(u32 *)(r1 +76)
  1: (61) r3 = *(u32 *)(r1 +80)
  2: (61) r7 = *(u32 *)(r1 +8)
  3: (bf) r0 = r2
  4: (07) r0 += 14
  5: (25) if r7 > 0x1 goto pc+4
   R0=pkt(id=0,off=14,r=0) R1=ctx R2=pkt(id=0,off=0,r=0)
   R3=pkt_end R7=inv,min_value=0,max_value=1 R10=fp
  6: (2d) if r0 > r3 goto pc+1
   R0=pkt(id=0,off=14,r=14) R1=ctx R2=pkt(id=0,off=0,r=14)
   R3=pkt_end R7=inv,min_value=0,max_value=1 R10=fp
  7: (63) *(u32 *)(r0 -4) = r0
  8: (b7) r0 = 0
  9: (95) exit

  from 6 to 8:
   R0=pkt(id=0,off=14,r=0) R1=ctx R2=pkt(id=0,off=0,r=0)
   R3=pkt_end R7=inv,min_value=0,max_value=1 R10=fp
  8: (b7) r0 = 0
  9: (95) exit

  from 5 to 10:
   R0=pkt(id=0,off=14,r=0) R1=ctx R2=pkt(id=0,off=0,r=0)
   R3=pkt_end R7=inv,min_value=2 R10=fp
  10: (07) r0 += 1
  11: (05) goto pc-6
  6: safe                           <----- here, wrongly found safe
  processed 15 insns

However, if we enforce a pruning mismatch by adding state into r8
which is then being mismatched in states_equal(), we find that for
the otherwise same program, the verifier detects a misaligned packet
access when actually walking that path:

  0: (61) r2 = *(u32 *)(r1 +76)
  1: (61) r3 = *(u32 *)(r1 +80)
  2: (61) r7 = *(u32 *)(r1 +8)
  3: (b7) r8 = 1
  4: (bf) r0 = r2
  5: (07) r0 += 14
  6: (25) if r7 > 0x1 goto pc+4
   R0=pkt(id=0,off=14,r=0) R1=ctx R2=pkt(id=0,off=0,r=0)
   R3=pkt_end R7=inv,min_value=0,max_value=1
   R8=imm1,min_value=1,max_value=1,min_align=1 R10=fp
  7: (2d) if r0 > r3 goto pc+1
   R0=pkt(id=0,off=14,r=14) R1=ctx R2=pkt(id=0,off=0,r=14)
   R3=pkt_end R7=inv,min_value=0,max_value=1
   R8=imm1,min_value=1,max_value=1,min_align=1 R10=fp
  8: (63) *(u32 *)(r0 -4) = r0
  9: (b7) r0 = 0
  10: (95) exit

  from 7 to 9:
   R0=pkt(id=0,off=14,r=0) R1=ctx R2=pkt(id=0,off=0,r=0)
   R3=pkt_end R7=inv,min_value=0,max_value=1
   R8=imm1,min_value=1,max_value=1,min_align=1 R10=fp
  9: (b7) r0 = 0
  10: (95) exit

  from 6 to 11:
   R0=pkt(id=0,off=14,r=0) R1=ctx R2=pkt(id=0,off=0,r=0)
   R3=pkt_end R7=inv,min_value=2
   R8=imm1,min_value=1,max_value=1,min_align=1 R10=fp
  11: (07) r0 += 1
  12: (b7) r8 = 0
  13: (05) goto pc-7                <----- mismatch due to r8
  7: (2d) if r0 > r3 goto pc+1
   R0=pkt(id=0,off=15,r=15) R1=ctx R2=pkt(id=0,off=0,r=15)
   R3=pkt_end R7=inv,min_value=2
   R8=imm0,min_value=0,max_value=0,min_align=2147483648 R10=fp
  8: (63) *(u32 *)(r0 -4) = r0
  misaligned packet access off 2+15+-4 size 4

The reason why we fail to see it in states_equal() is that the
third test in compare_ptrs_to_packet() ...

  if (old->off <= cur->off &&
      old->off >= old->range && cur->off >= cur->range)
          return true;

... will let the above pass. The situation we run into is that
old->off <= cur->off (14 <= 15), meaning that prior walked paths
went with smaller offset, which was later used in the packet
access after successful packet range check and found to be safe
already.

For example: Given is R0=pkt(id=0,off=0,r=0). Adding offset 14
as in above program to it, results in R0=pkt(id=0,off=14,r=0)
before the packet range test. Now, testing this against R3=pkt_end
with 'if r0 > r3 goto out' will transform R0 into R0=pkt(id=0,off=14,r=14)
for the case when we're within bounds. A write into the packet
at offset *(u32 *)(r0 -4), that is, 2 + 14 -4, is valid and
aligned (2 is for NET_IP_ALIGN). After processing this with
all fall-through paths, we later on check paths from branches.
When the above skb->mark test is true, then we jump near the
end of the program, perform r0 += 1, and jump back to the
'if r0 > r3 goto out' test we've visited earlier already. This
time, R0 is of type R0=pkt(id=0,off=15,r=0), and we'll prune
that part because this time we'll have a larger safe packet
range, and we already found that with off=14 all further insn
were already safe, so it's safe as well with a larger off.
However, the problem is that the subsequent write into the packet
with 2 + 15 -4 is then unaligned, and not caught by the alignment
tracking. Note that min_align, aux_off, and aux_off_align were
all 0 in this example.

Since we cannot tell at this time what kind of packet access was
performed in the prior walk and what minimal requirements it has
(we might do so in the future, but that requires more complexity),
fix it to disable this pruning case for strict alignment for now,
and let the verifier do check such paths instead. With that applied,
the test cases pass and reject the program due to misalignment.

Fixes: d1174416747d ("bpf: Track alignment of register values in the verifier.")
Reference: http://patchwork.ozlabs.org/patch/761909/
Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: David S. Miller

net: Make IP alignment calulations clearer.

2017-05-22T16:27:07+00:00

The assignmnet:

	ip_align = strict ? 2 : NET_IP_ALIGN;

in compare_pkt_ptr_alignment() trips up Coverity because we can only
get to this code when strict is true, therefore ip_align will always
be 2 regardless of NET_IP_ALIGN's value.

So just assign directly to '2' and explain the situation in the
comment above.

Reported-by: "Gustavo A. R. Silva" 
Signed-off-by: David S. Miller

bpf: adjust verifier heuristics

2017-05-18T02:55:27+00:00

Current limits with regards to processing program paths do not
really reflect today's needs anymore due to programs becoming
more complex and verifier smarter, keeping track of more data
such as const ALU operations, alignment tracking, spilling of
PTR_TO_MAP_VALUE_ADJ registers, and other features allowing for
smarter matching of what LLVM generates.

This also comes with the side-effect that we result in fewer
opportunities to prune search states and thus often need to do
more work to prove safety than in the past due to different
register states and stack layout where we mismatch. Generally,
it's quite hard to determine what caused a sudden increase in
complexity, it could be caused by something as trivial as a
single branch somewhere at the beginning of the program where
LLVM assigned a stack slot that is marked differently throughout
other branches and thus causing a mismatch, where verifier
then needs to prove safety for the whole rest of the program.
Subsequently, programs with even less than half the insn size
limit can get rejected. We noticed that while some programs
load fine under pre 4.11, they get rejected due to hitting
limits on more recent kernels. We saw that in the vast majority
of cases (90+%) pruning failed due to register mismatches. In
case of stack mismatches, majority of cases failed due to
different stack slot types (invalid, spill, misc) rather than
differences in spilled registers.

This patch makes pruning more aggressive by also adding markers
that sit at conditional jumps as well. Currently, we only mark
jump targets for pruning. For example in direct packet access,
these are usually error paths where we bail out. We found that
adding these markers, it can reduce number of processed insns
by up to 30%. Another option is to ignore reg->id in probing
PTR_TO_MAP_VALUE_OR_NULL registers, which can help pruning
slightly as well by up to 7% observed complexity reduction as
stand-alone. Meaning, if a previous path with register type
PTR_TO_MAP_VALUE_OR_NULL for map X was found to be safe, then
in the current state a PTR_TO_MAP_VALUE_OR_NULL register for
the same map X must be safe as well. Last but not least the
patch also adds a scheduling point and bumps the current limit
for instructions to be processed to a more adequate value.

Signed-off-by: Daniel Borkmann 
Acked-by: Alexei Starovoitov 
Signed-off-by: David S. Miller

bpf: Handle multiple variable additions into packet pointers in verifier.

2017-05-12T02:48:58+00:00

We must accumulate into reg->aux_off rather than use a plain assignment.

Add a test for this situation to test_align.

Reported-by: Alexei Starovoitov 
Signed-off-by: David S. Miller

bpf: Add strict alignment flag for BPF_PROG_LOAD.

2017-05-11T18:19:00+00:00

Add a new field, "prog_flags", and an initial flag value
BPF_F_STRICT_ALIGNMENT.

When set, the verifier will enforce strict pointer alignment
regardless of the setting of CONFIG_EFFICIENT_UNALIGNED_ACCESS.

The verifier, in this mode, will also use a fixed value of "2" in
place of NET_IP_ALIGN.

This facilitates test cases that will exercise and validate this part
of the verifier even when run on architectures where alignment doesn't
matter.

Signed-off-by: David S. Miller 
Acked-by: Daniel Borkmann

bpf: Do per-instruction state dumping in verifier when log_level > 1.

2017-05-11T18:19:00+00:00

If log_level > 1, do a state dump every instruction and emit it in
a more compact way (without a leading newline).

This will facilitate more sophisticated test cases which inspect the
verifier log for register state.

Signed-off-by: David S. Miller 
Acked-by: Daniel Borkmann

bpf: Track alignment of register values in the verifier.

2017-05-11T18:19:00+00:00

Currently if we add only constant values to pointers we can fully
validate the alignment, and properly check if we need to reject the
program on !CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS architectures.

However, once an unknown value is introduced we only allow byte sized
memory accesses which is too restrictive.

Add logic to track the known minimum alignment of register values,
and propagate this state into registers containing pointers.

The most common paradigm that makes use of this new logic is computing
the transport header using the IP header length field.  For example:

	struct ethhdr *ep = skb->data;
	struct iphdr *iph = (struct iphdr *) (ep + 1);
	struct tcphdr *th;
 ...
	n = iph->ihl;
	th = ((void *)iph + (n * 4));
	port = th->dest;

The existing code will reject the load of th->dest because it cannot
validate that the alignment is at least 2 once "n * 4" is added the
the packet pointer.

In the new code, the register holding "n * 4" will have a reg->min_align
value of 4, because any value multiplied by 4 will be at least 4 byte
aligned.  (actually, the eBPF code emitted by the compiler in this case
is most likely to use a shift left by 2, but the end result is identical)

At the critical addition:

	th = ((void *)iph + (n * 4));

The register holding 'th' will start with reg->off value of 14.  The
pointer addition will transform that reg into something that looks like:

	reg->aux_off = 14
	reg->aux_off_align = 4

Next, the verifier will look at the th->dest load, and it will see
a load offset of 2, and first check:

	if (reg->aux_off_align % size)

which will pass because aux_off_align is 4.  reg_off will be computed:

	reg_off = reg->off;
 ...
		reg_off += reg->aux_off;

plus we have off==2, and it will thus check:

	if ((NET_IP_ALIGN + reg_off + off) % size != 0)

which evaluates to:

	if ((NET_IP_ALIGN + 14 + 2) % size != 0)

On strict alignment architectures, NET_IP_ALIGN is 2, thus:

	if ((2 + 14 + 2) % size != 0)

which passes.

These pointer transformations and checks work regardless of whether
the constant offset or the variable with known alignment is added
first to the pointer register.

Signed-off-by: David S. Miller 
Acked-by: Daniel Borkmann