Re: [PATCH bpf-next 3/3] bpf: relax MUL range computation check

[Yonghong Song <yonghong.song@linux.dev>]

On 4/11/24 10:37 AM, Cupertino Miranda wrote:
> MUL instruction required that src_reg would be a known value (i.e.
> src_reg would be evaluate as a const value). The condition in this case
> can be relaxed, since multiplication is a commutative operator and the
> range computation is still valid if at least one of its registers is
> known.
>
> BPF self-tests were added to check the new functionality.
>
> Signed-off-by: Cupertino Miranda <cupertino.miranda@oracle.com>
> ---
>   kernel/bpf/verifier.c                         | 10 +-
>   .../selftests/bpf/progs/verifier_bounds.c     | 99 +++++++++++++++++++
>   2 files changed, 105 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7894af2e1bdb..a326ec024d82 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -13741,15 +13741,17 @@ static bool is_const_reg_and_valid(struct bpf_reg_state reg, bool alu32,
>   }
>   
>   static bool is_safe_to_compute_dst_reg_ranges(struct bpf_insn *insn,
> +					      struct bpf_reg_state dst_reg,
>   					      struct bpf_reg_state src_reg)
>   {
> -	bool src_known;
> +	bool src_known, dst_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);
> +	dst_known = is_const_reg_and_valid(dst_reg, alu32, &valid_known);

Is it a possible the above could falsely reject some operation since
in the original code, dst_reg is not checked here?

>   
>   	/* Taint dst register if offset had invalid bounds
>   	 * derived from e.g. dead branches.
> @@ -13765,10 +13767,10 @@ static bool is_safe_to_compute_dst_reg_ranges(struct bpf_insn *insn,
>   	case BPF_OR:
>   		return true;
>   
> -	/* Compute range for MUL if the src_reg is known.
> +	/* Compute range for MUL if at least one of its registers is know.

know => known

>   	 */
>   	case BPF_MUL:
> -		return src_known;
> +		return src_known || dst_known;
>   
>   	/* Shift operators range is only computable if shift dimension operand
>   	 * is known. Also, shifts greater than 31 or 63 are undefined. This
> @@ -13799,7 +13801,7 @@ static int adjust_scalar_min_max_vals(struct bpf_verifier_env *env,
>   	bool alu32 = (BPF_CLASS(insn->code) != BPF_ALU64);
>   	int ret;
>   
> -	if (!is_safe_to_compute_dst_reg_ranges(insn, src_reg)) {
> +	if (!is_safe_to_compute_dst_reg_ranges(insn, *dst_reg, src_reg)) {
>   		__mark_reg_unknown(env, dst_reg);
>   		return 0;
>   	}
> diff --git a/tools/testing/selftests/bpf/progs/verifier_bounds.c b/tools/testing/selftests/bpf/progs/verifier_bounds.c

Let us break this commit into two patches: verifier change and selftest change. This will make possible backport easier.

> index 2fcf46341b30..09bb1b270ca7 100644
> --- a/tools/testing/selftests/bpf/progs/verifier_bounds.c
> +++ b/tools/testing/selftests/bpf/progs/verifier_bounds.c
> @@ -949,6 +949,105 @@ l1_%=:	r0 = 0;						\
>   	: __clobber_all);
>   }
>   
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