tree 14f8a4ceea46af539056ea47e926eb5e5b044ad5
parent 3a71dc366d4aa51a22f385445f2862231d3fda3b
author John Fastabend <john.fastabend@gmail.com> 1590773339 -0700
committer Alexei Starovoitov <ast@kernel.org> 1590784446 -0700

bpf, selftests: Verifier bounds tests need to be updated

After previous fix for zero extension test_verifier tests #65 and #66 now
fail. Before the fix we can see the alu32 mov op at insn 10

10: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=invP(id=0,
              smin_value=4294967168,smax_value=4294967423,
              umin_value=4294967168,umax_value=4294967423,
              var_off=(0x0; 0x1ffffffff),
              s32_min_value=-2147483648,s32_max_value=2147483647,
              u32_min_value=0,u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm
10: (bc) w1 = w1
11: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=invP(id=0,
              smin_value=0,smax_value=2147483647,
              umin_value=0,umax_value=4294967295,
              var_off=(0x0; 0xffffffff),
              s32_min_value=-2147483648,s32_max_value=2147483647,
              u32_min_value=0,u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm

After the fix at insn 10 because we have 's32_min_value < 0' the following
step 11 now has 'smax_value=U32_MAX' where before we pulled the s32_max_value
bound into the smax_value as seen above in 11 with smax_value=2147483647.

10: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=inv(id=0,
             smin_value=4294967168,smax_value=4294967423,
             umin_value=4294967168,umax_value=4294967423,
             var_off=(0x0; 0x1ffffffff),
             s32_min_value=-2147483648, s32_max_value=2147483647,
             u32_min_value=0,u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm
10: (bc) w1 = w1
11: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=inv(id=0,
             smin_value=0,smax_value=4294967295,
             umin_value=0,umax_value=4294967295,
             var_off=(0x0; 0xffffffff),
             s32_min_value=-2147483648, s32_max_value=2147483647,
             u32_min_value=0, u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm

The fall out of this is by the time we get to the failing instruction at
step 14 where previously we had the following:

14: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=inv(id=0,
             smin_value=72057594021150720,smax_value=72057594029539328,
             umin_value=72057594021150720,umax_value=72057594029539328,
             var_off=(0xffffffff000000; 0xffffff),
             s32_min_value=-16777216,s32_max_value=-1,
             u32_min_value=-16777216,u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm
14: (0f) r0 += r1

We now have,

14: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=inv(id=0,
             smin_value=0,smax_value=72057594037927935,
             umin_value=0,umax_value=72057594037927935,
             var_off=(0x0; 0xffffffffffffff),
             s32_min_value=-2147483648,s32_max_value=2147483647,
             u32_min_value=0,u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm
14: (0f) r0 += r1

In the original step 14 'smin_value=72057594021150720' this trips the logic
in the verifier function check_reg_sane_offset(),

 if (smin >= BPF_MAX_VAR_OFF || smin <= -BPF_MAX_VAR_OFF) {
	verbose(env, "value %lld makes %s pointer be out of bounds\n",
		smin, reg_type_str[type]);
	return false;
 }

Specifically, the 'smin <= -BPF_MAX_VAR_OFF' check. But with the fix
at step 14 we have bounds 'smin_value=0' so the above check is not tripped
because BPF_MAX_VAR_OFF=1<<29.

We have a smin_value=0 here because at step 10 the smaller smin_value=0 means
the subtractions at steps 11 and 12 bring the smin_value negative.

11: (17) r1 -= 2147483584
12: (17) r1 -= 2147483584
13: (77) r1 >>= 8

Then the shift clears the top bit and smin_value is set to 0. Note we still
have the smax_value in the fixed code so any reads will fail. An alternative
would be to have reg_sane_check() do both smin and smax value tests.

To fix the test we can omit the 'r1 >>=8' at line 13. This will change the
err string, but keeps the intention of the test as suggseted by the title,
"check after truncation of boundary-crossing range". If the verifier logic
changes a different value is likely to be thrown in the error or the error
will no longer be thrown forcing this test to be examined. With this change
we see the new state at step 13.

13: R0_w=map_value(id=0,off=0,ks=8,vs=8,imm=0)
    R1_w=invP(id=0,
              smin_value=-4294967168,smax_value=127,
              umin_value=0,umax_value=18446744073709551615,
              s32_min_value=-2147483648,s32_max_value=2147483647,
              u32_min_value=0,u32_max_value=-1)
    R10=fp0 fp-8_w=mmmmmmmm

Giving the expected out of bounds error, "value -4294967168 makes map_value
pointer be out of bounds" However, for unpriv case we see a different error
now because of the mixed signed bounds pointer arithmatic. This seems OK so
I've only added the unpriv_errstr for this. Another optino may have been to
do addition on r1 instead of subtraction but I favor the approach above
slightly.

Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/159077333942.6014.14004320043595756079.stgit@john-Precision-5820-Tower