Hi,
I've just written a BPF verifier fuzzer, targeting logic bugs in the
BPF verifier.
The following is an abnormal case it generated. The case only contains 10
BPF instructions but costs more than 2 mins to load on :
HEAD commit: f0c4d9fc9cc9 Linux 6.1-rc4
git tree: upstream
kernel config: https://pastebin.com/raw/SBxaikiG
C reproducer: https://pastebin.com/raw/HsDXdraZ
verifier log: https://pastebin.com/raw/sNmSsVxs
Ideally, the verifier should exit quickly in this case, since R2=42
always holds.
The behaviour of the verifier does not make sense to me, seems it lost
the range information of R2.
Please point out if I missed anything, the C reproducer in the link
(https://pastebin.com/raw/HsDXdraZ)
essentially loads the following case into `test_verifier.c`:
{
"BVF verifier test",
.insns = {
BPF_MOV64_IMM(BPF_REG_1, 42),
BPF_MOV64_IMM(BPF_REG_2, 0),
BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 1, 0, 1),
BPF_EXIT_INSN(),
BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_1),
BPF_ALU32_IMM(BPF_DIV, BPF_REG_2, 1),
BPF_ALU64_IMM(BPF_SUB, BPF_REG_1, 108),
BPF_JMP32_REG(BPF_JGT, BPF_REG_1, BPF_REG_2, -3),
BPF_MOV64_REG(BPF_REG_0, BPF_REG_2),
BPF_EXIT_INSN(),
},
.prog_type = BPF_PROG_TYPE_XDP,
},
The verifier's log is more then 4M, but essentially is:
0: R1=ctx(off=0,imm=0) R10=fp0
0: (b7) r1 = 42 ; R1_w=P42
1: (b7) r2 = 0 ; R2_w=P0
2: (85) call pc+1
caller:
R10=fp0
callee:
frame1: R1_w=P42 R2_w=P0 R10=fp0
4: (57) r2 &= -52 ; frame1: R2_w=P0
5: (0f) r2 += r1 ; frame1: R1_w=P42 R2_w=P42
6: (34) w2 /= 1 ; frame1:
R2_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff))
7: (17) r1 -= 108 ; frame1: R1_w=P-66
8: (2e) if w1 > w2 goto pc-3 6: frame1: R1_w=P-66
R2_w=Pscalar(umax=4294967229,var_off=(0x0; 0xffffffff)) R10=fp0
6: (34) w2 /= 1 ; frame1:
R2_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff))
7: (17) r1 -= 108 ; frame1: R1_w=P-174
8: (2e) if w1 > w2 goto pc-3 6: frame1: R1_w=P-174
R2_w=Pscalar(umax=4294967121,var_off=(0x0; 0xffffffff)) R10=fp0
6: (34) w2 /= 1 ; frame1:
R2_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff))
7: (17) r1 -= 108 ; frame1: R1=P-282
8: (2e) if w1 > w2 goto pc-3 6: frame1: R1=P-282
R2=Pscalar(umax=4294967013,var_off=(0x0; 0xffffffff)) R10=fp0
...
6: (34) w2 /= 1 ; frame1:
R2_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff))
7: (17) r1 -= 108 ; frame1: R1_w=P-6342690
8: (2e) if w1 > w2 goto pc-3 6: frame1: R1_w=P-6342690
R2_w=Pscalar(umax=4288624605,var_off=(0x0; 0xffffffff)) R10=fp0
6: (34) w2 /= 1 ; frame1:
R2_w=Pscalar(umax=4294967295,var_off=(0x0; 0xffffffff))
7: (17) r1 -= 108 ; frame1: R1_w=P-6342798
8: (2e) if w1 > w2 goto pc-3 ; frame1: R1_w=P-6342798
R2_w=Pscalar(umin=4288624498,umax=4294967295,var_off=(0xff800000;
0x7fffff),s32_min=-6342798,s32_max=-1)
9: (bf) r0 = r2 ; frame1:
R0_w=Pscalar(id=58730,umin=4288624498,umax=4294967295,var_off=(0xff800000;
0x7fffff),s32_min=-6342798,s32_max=-1)
R2_w=Pscalar(id=58730,umin=4288624498,umax=4294967295,var_off=(0xff800000;
0x7fffff),s32_min=-6342798,s32_max=-1)
10: (95) exit
returning from callee:
frame1: R0_w=Pscalar(id=58730,umin=4288624498,umax=4294967295,var_off=(0xff800000;
0x7fffff),s32_min=-6342798,s32_max=-1) R1_w=P-6342798
R2_w=Pscalar(id=58730,umin=4288624498,umax=4294967295,var_off=(0xff800000;
0x7fffff),s32_min=-6342798,s32_max=-1) R10=fp0
to caller at 3:
R0_w=Pscalar(id=58730,umin=4288624498,umax=429496
