Subject: Re: Regression fix for bpf in v5.3 (was Re: [RFC PATCH] bpf: handle 32-bit zext during constant blinding)

From: Jiong Wang <[email-masked]>
Date: Wed, 21 Aug 2019 11:55:54 +0100

Michael Ellerman writes:

> "Naveen N. Rao" <[email-masked]> writes:
>> Since BPF constant blinding is performed after the verifier pass, there
>> are certain ALU32 instructions inserted which don't have a corresponding
>> zext instruction inserted after. This is causing a kernel oops on
>> powerpc and can be reproduced by running 'test_cgroup_storage' with
>> bpf_jit_harden=2.
>>
>> Fix this by emitting BPF_ZEXT during constant blinding if
>> prog->aux->verifier_zext is set.
>>
>> Fixes: a4b1d3c1ddf6cb ("bpf: verifier: insert zero extension according to analysis result")
>> Reported-by: Michael Ellerman <[email-masked]>
>> Signed-off-by: Naveen N. Rao <[email-masked]>
>> ---
>> This approach (the location where zext is being introduced below, in 
>> particular) works for powerpc, but I am not entirely sure if this is 
>> sufficient for other architectures as well. This is broken on v5.3-rc4.
>
> Any comment on this?

Have commented on https://marc.info/?l=linux-netdev&m=156637836024743&w=2

The fix looks correct to me on "BPF_LD | BPF_IMM | BPF_DW", but looks
unnecessary on two other places. It would be great if you or Naveen could
confirm it.

Thanks.

Regards,
Jiong

> This is a regression in v5.3, which results in a kernel crash, it would
> be nice to get it fixed before the release please?
>
> cheers
>
>> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
>> index 8191a7db2777..d84146e6fd9e 100644
>> --- a/kernel/bpf/core.c
>> +++ b/kernel/bpf/core.c
>> @@ -890,7 +890,8 @@ int bpf_jit_get_func_addr(const struct bpf_prog *prog,
>>  
>>  static int bpf_jit_blind_insn(const struct bpf_insn *from,
>>  			      const struct bpf_insn *aux,
>> -			      struct bpf_insn *to_buff)
>> +			      struct bpf_insn *to_buff,
>> +			      bool emit_zext)
>>  {
>>  	struct bpf_insn *to = to_buff;
>>  	u32 imm_rnd = get_random_int();
>> @@ -939,6 +940,8 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
>>  		*to++ = BPF_ALU32_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^ from->imm);
>>  		*to++ = BPF_ALU32_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
>>  		*to++ = BPF_ALU32_REG(from->code, from->dst_reg, BPF_REG_AX);
>> +		if (emit_zext)
>> +			*to++ = BPF_ZEXT_REG(from->dst_reg);
>>  		break;
>>  
>>  	case BPF_ALU64 | BPF_ADD | BPF_K:
>> @@ -992,6 +995,10 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
>>  			off -= 2;
>>  		*to++ = BPF_ALU32_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^ from->imm);
>>  		*to++ = BPF_ALU32_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
>> +		if (emit_zext) {
>> +			*to++ = BPF_ZEXT_REG(BPF_REG_AX);
>> +			off--;
>> +		}
>>  		*to++ = BPF_JMP32_REG(from->code, from->dst_reg, BPF_REG_AX,
>>  				      off);
>>  		break;
>> @@ -1005,6 +1012,8 @@ static int bpf_jit_blind_insn(const struct bpf_insn *from,
>>  	case 0: /* Part 2 of BPF_LD | BPF_IMM | BPF_DW. */
>>  		*to++ = BPF_ALU32_IMM(BPF_MOV, BPF_REG_AX, imm_rnd ^ aux[0].imm);
>>  		*to++ = BPF_ALU32_IMM(BPF_XOR, BPF_REG_AX, imm_rnd);
>> +		if (emit_zext)
>> +			*to++ = BPF_ZEXT_REG(BPF_REG_AX);
>>  		*to++ = BPF_ALU64_REG(BPF_OR,  aux[0].dst_reg, BPF_REG_AX);
>>  		break;
>>  
>> @@ -1088,7 +1097,8 @@ struct bpf_prog *bpf_jit_blind_constants(struct bpf_prog *prog)
>>  		    insn[1].code == 0)
>>  			memcpy(aux, insn, sizeof(aux));
>>  
>> -		rewritten = bpf_jit_blind_insn(insn, aux, insn_buff);
>> +		rewritten = bpf_jit_blind_insn(insn, aux, insn_buff,
>> +						clone->aux->verifier_zext);
>>  		if (!rewritten)
>>  			continue;
>>  
>> -- 
>> 2.22.0