Re: Fwd: [PATCH bpf] bpf: Do more tight ALU bounds tracking

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On 8/8/22 5:14 PM, Kuee k1r0a wrote:
---------- Forwarded message ---------
From: Kuee k1r0a <liulin063@xxxxxxxxx>
Date: Mon, Aug 8, 2022 at 11:11 PM
Subject: Re: [PATCH bpf] bpf: Do more tight ALU bounds tracking
To: Daniel Borkmann <daniel@xxxxxxxxxxxxx>


On Mon, Aug 8, 2022 at 9:25 PM Daniel Borkmann <daniel@xxxxxxxxxxxxx> wrote:

On 7/30/22 12:48 AM, Hao Luo wrote:
On Fri, Jul 29, 2022 at 3:43 PM Youlin Li <liulin063@xxxxxxxxx> wrote:

In adjust_scalar_min_max_vals(), let 32bit bounds learn from 64bit bounds
to get more tight bounds tracking. Similar operation can be found in
reg_set_min_max().

Also, we can now fold reg_bounds_sync() into zext_32_to_64().

Before:

      func#0 @0
      0: R1=ctx(off=0,imm=0) R10=fp0
      0: (b7) r0 = 0                        ; R0_w=0
      1: (b7) r1 = 0                        ; R1_w=0
      2: (87) r1 = -r1                      ; R1_w=scalar()
      3: (87) r1 = -r1                      ; R1_w=scalar()
      4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
      5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0xffffffff))  <--- [*]
      6: (95) exit

It can be seen that even if the 64bit bounds is clear here, the 32bit
bounds is still in the state of 'UNKNOWN'.

After:

      func#0 @0
      0: R1=ctx(off=0,imm=0) R10=fp0
      0: (b7) r0 = 0                        ; R0_w=0
      1: (b7) r1 = 0                        ; R1_w=0
      2: (87) r1 = -r1                      ; R1_w=scalar()
      3: (87) r1 = -r1                      ; R1_w=scalar()
      4: (c7) r1 s>>= 63                    ; R1_w=scalar(smin=-1,smax=0)
      5: (07) r1 += 2                       ; R1_w=scalar(umin=1,umax=2,var_off=(0x0; 0x3))  <--- [*]
      6: (95) exit

Signed-off-by: Youlin Li <liulin063@xxxxxxxxx>

Looks good to me. Thanks Youlin.

Acked-by: Hao Luo <haoluo@xxxxxxxxxx>

Thanks Youlin! Looks like the patch breaks CI [0] e.g.:

    #142/p bounds check after truncation of non-boundary-crossing range FAIL
    Failed to load prog 'Permission denied'!
    invalid access to map value, value_size=8 off=16777215 size=1
    R0 max value is outside of the allowed memory range
    verification time 296 usec
    stack depth 8
    processed 15 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0

Please take a look. Also it would be great to add a test_verifier selftest to
assert above case from commit log against future changes.

Thanks,
Daniel

    [0] https://github.com/kernel-patches/bpf/runs/7696324041?check_suite_focus=true

This test case fails because the 32bit boundary information is lost
after the 11th instruction is executed:
Before:
     11: (07) r1 += 2147483647             ;
R1_w=scalar(umin=70866960383,umax=70866960638,var_off=(0x1000000000;
0xffffffff),u32_min=2147483647,u32_max=-2147483394)
After:
     11: (07) r1 += 2147483647             ;
R1_w=scalar(umin=70866960383,umax=70866960638,var_off=(0x1000000000;
0xffffffff))

This may be because, in previous versions of the code, when
__reg_combine_64_into_32() was called, the 32bit boundary was
completely deduced from the 64bit boundary, so there was a call to
__mark_reg32_unbounded() in __reg_combine_64_into_32().

But now, before adjust_scalar_min_max_vals() calls
__reg_combine_64_into_32() , the 32bit bounds are already calculated
to some extent, and __mark_reg32_unbounded() will eliminate these
information.

Simply copying a code without __mark_reg32_unbounded() should work,
perhaps it would be more elegant to introduce a flag into
__reg_combine_64_into_32()?

Sorry for not completing the tests because I did not 'make selftests'
successfully, and uploaded the code that caused the error.

Under tools/testing/selftests/bpf/, you can run test_progs and test_verifier
through the vmtest script, e.g. `./vmtest.sh -- ./test_progs` should ease
running it. The whole `make selftests` is not necessary given here we care
about BPF, CI is running these where 2 failed and need investigation:

          test_progs: PASS
 test_progs-no_alu32: FAIL (returned 1)
           test_maps: PASS
       test_verifier: FAIL (returned 1)

Fwiw, for the test_verifier failure case at least, we should then adapt it
in a separate commit with an analysis explaining why it is okay to alter the
test; plus a 3rd commit adding new test cases as mentioned earlier.

Thanks a lot, Kuee!
Daniel



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