Re: [PATCH bpf-next 2/3] bpf: refactor checks for range computation

[Cupertino Miranda <cupertino.miranda@oracle.com>]

Yonghong Song writes:

> On 4/11/24 10:37 AM, Cupertino Miranda wrote:
>> Split range computation checks in its own function, isolating pessimitic
>> range set for dst_reg and failing return to a single point.
>>
>> Signed-off-by: Cupertino Miranda <cupertino.miranda@oracle.com>
>> ---
>>   kernel/bpf/verifier.c | 141 +++++++++++++++++++++++-------------------
>>   1 file changed, 77 insertions(+), 64 deletions(-)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index a219f601569a..7894af2e1bdb 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -13709,6 +13709,82 @@ static void scalar_min_max_arsh(struct bpf_reg_state *dst_reg,
>>   	__update_reg_bounds(dst_reg);
>>   }
>>   +static bool is_const_reg_and_valid(struct bpf_reg_state reg, bool alu32,
>> +				   bool *valid)
>> +{
>> +	s64 smin_val = reg.smin_value;
>> +	s64 smax_val = reg.smax_value;
>> +	u64 umin_val = reg.umin_value;
>> +	u64 umax_val = reg.umax_value;
>> +
>> +	s32 s32_min_val = reg.s32_min_value;
>> +	s32 s32_max_val = reg.s32_max_value;
>> +	u32 u32_min_val = reg.u32_min_value;
>> +	u32 u32_max_val = reg.u32_max_value;
>> +
>> +	bool known = alu32 ? tnum_subreg_is_const(reg.var_off) :
>> +			     tnum_is_const(reg.var_off);
>> +
>> +	if (alu32) {
>> +		if ((known &&
>> +		     (s32_min_val != s32_max_val || u32_min_val != u32_max_val)) ||
>> +		      s32_min_val > s32_max_val || u32_min_val > u32_max_val)
>> +			*valid &= false;
>
> *valid = false;
>
>> +	} else {
>> +		if ((known &&
>> +		     (smin_val != smax_val || umin_val != umax_val)) ||
>> +		    smin_val > smax_val || umin_val > umax_val)
>> +			*valid &= false;
>
> *valid = false;
>
>> +	}
>> +
>> +	return known;
>> +}
>> +
>> +static bool is_safe_to_compute_dst_reg_ranges(struct bpf_insn *insn,
>> +					      struct bpf_reg_state src_reg)
>> +{
>> +	bool src_known;
>> +	u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
>> +	bool alu32 = (BPF_CLASS(insn->code) != BPF_ALU64);
>> +	u8 opcode = BPF_OP(insn->code);
>> +
>> +	bool valid_known = true;
>> +	src_known = is_const_reg_and_valid(src_reg, alu32, &valid_known);
>> +
>> +	/* Taint dst register if offset had invalid bounds
>> +	 * derived from e.g. dead branches.
>> +	 */
>> +	if (valid_known == false)
>> +		return false;
>> +
>> +	switch (opcode) {
>> +	case BPF_ADD:
>> +	case BPF_SUB:
>> +	case BPF_AND:
>> +	case BPF_XOR:
>> +	case BPF_OR:
>> +		return true;
>> +
>> +	/* Compute range for MUL if the src_reg is known.
>> +	 */
>> +	case BPF_MUL:
>> +		return src_known;
>> +
>> +	/* Shift operators range is only computable if shift dimension operand
>> +	 * is known. Also, shifts greater than 31 or 63 are undefined. This
>> +	 * includes shifts by a negative number.
>> +	 */
>> +	case BPF_LSH:
>> +	case BPF_RSH:
>> +	case BPF_ARSH:
>> +		return src_known && (src_reg.umax_value < insn_bitness);
>> +	default:
>> +		break;
>> +	}
>> +
>> +	return false;
>> +}
>> +
>>   /* WARNING: This function does calculations on 64-bit values, but the actual
>>    * execution may occur on 32-bit values. Therefore, things like bitshifts
>>    * need extra checks in the 32-bit case.
>> @@ -13720,52 +13796,10 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>>   {
>>   	struct bpf_reg_state *regs = cur_regs(env);
>>   	u8 opcode = BPF_OP(insn->code);
>> -	bool src_known;
>> -	s64 smin_val, smax_val;
>> -	u64 umin_val, umax_val;
>> -	s32 s32_min_val, s32_max_val;
>> -	u32 u32_min_val, u32_max_val;
>> -	u64 insn_bitness = (BPF_CLASS(insn->code) == BPF_ALU64) ? 64 : 32;
>>   	bool alu32 = (BPF_CLASS(insn->code) != BPF_ALU64);
>>   	int ret;
>>   -	smin_val = src_reg.smin_value;
>> -	smax_val = src_reg.smax_value;
>> -	umin_val = src_reg.umin_value;
>> -	umax_val = src_reg.umax_value;
>> -
>> -	s32_min_val = src_reg.s32_min_value;
>> -	s32_max_val = src_reg.s32_max_value;
>> -	u32_min_val = src_reg.u32_min_value;
>> -	u32_max_val = src_reg.u32_max_value;
>> -
>> -	if (alu32) {
>> -		src_known = tnum_subreg_is_const(src_reg.var_off);
>> -		if ((src_known &&
>> -		     (s32_min_val != s32_max_val || u32_min_val != u32_max_val)) ||
>> -		    s32_min_val > s32_max_val || u32_min_val > u32_max_val) {
>> -			/* Taint dst register if offset had invalid bounds
>> -			 * derived from e.g. dead branches.
>> -			 */
>> -			__mark_reg_unknown(env, dst_reg);
>> -			return 0;
>> -		}
>> -	} else {
>> -		src_known = tnum_is_const(src_reg.var_off);
>> -		if ((src_known &&
>> -		     (smin_val != smax_val || umin_val != umax_val)) ||
>> -		    smin_val > smax_val || umin_val > umax_val) {
>> -			/* Taint dst register if offset had invalid bounds
>> -			 * derived from e.g. dead branches.
>> -			 */
>> -			__mark_reg_unknown(env, dst_reg);
>> -			return 0;
>> -		}
>> -	}
>> -
>> -	if (!src_known &&
>> -	    opcode != BPF_ADD && opcode != BPF_SUB && opcode != BPF_AND &&
>> -	    opcode != BPF_XOR && opcode != BPF_OR) {
>> +	if (!is_safe_to_compute_dst_reg_ranges(insn, src_reg)) {
>>   		__mark_reg_unknown(env, dst_reg);
>
> This is not a precise refactoring. there are some cases like below
> which uses mark_reg_unknow().
Oh, right I miss those underscores above. :(
Will make sure to cover that.
>
> Let us put the refactoring patch as the first patch in the serious and all
> additional changes after that and this will make it easy to review.
>
>>   		return 0;
>>   	}
>> @@ -13822,39 +13856,18 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>>   		scalar_min_max_xor(dst_reg, &src_reg);
>>   		break;
>>   	case BPF_LSH:
>> -		if (umax_val >= insn_bitness) {
>> -			/* Shifts greater than 31 or 63 are undefined.
>> -			 * This includes shifts by a negative number.
>> -			 */
>> -			mark_reg_unknown(env, regs, insn->dst_reg);
>> -			break;
>> -		}
>>   		if (alu32)
>>   			scalar32_min_max_lsh(dst_reg, &src_reg);
>>   		else
>>   			scalar_min_max_lsh(dst_reg, &src_reg);
>>   		break;
>>   	case BPF_RSH:
>> -		if (umax_val >= insn_bitness) {
>> -			/* Shifts greater than 31 or 63 are undefined.
>> -			 * This includes shifts by a negative number.
>> -			 */
>> -			mark_reg_unknown(env, regs, insn->dst_reg);
>> -			break;
>> -		}
>>   		if (alu32)
>>   			scalar32_min_max_rsh(dst_reg, &src_reg);
>>   		else
>>   			scalar_min_max_rsh(dst_reg, &src_reg);
>>   		break;
>>   	case BPF_ARSH:
>> -		if (umax_val >= insn_bitness) {
>> -			/* Shifts greater than 31 or 63 are undefined.
>> -			 * This includes shifts by a negative number.
>> -			 */
>> -			mark_reg_unknown(env, regs, insn->dst_reg);
>> -			break;
>> -		}
>>   		if (alu32)
>>   			scalar32_min_max_arsh(dst_reg, &src_reg);
>>   		else