netdev - Re: [PATCH bpf-next] bpf: Emit explicit NULL pointer checks for PROBE

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Message-ID: <A746402C-245A-4FF1-AB54-585537EEBA9B@fb.com>
Date:   Wed, 3 Feb 2021 00:56:39 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next] bpf: Emit explicit NULL pointer checks for
 PROBE_LDX instructions.



> On Feb 1, 2021, at 9:38 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> From: Alexei Starovoitov <ast@...nel.org>
> 
> PTR_TO_BTF_ID registers contain either kernel pointer or NULL.
> Emit the NULL check explicitly by JIT instead of going into
> do_user_addr_fault() on NULL deference.
> 
> Signed-off-by: Alexei Starovoitov <ast@...nel.org>
> ---
> arch/x86/net/bpf_jit_comp.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
> 
> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
> index b7a2911bda77..a3dc3bd154ac 100644
> --- a/arch/x86/net/bpf_jit_comp.c
> +++ b/arch/x86/net/bpf_jit_comp.c
> @@ -930,6 +930,7 @@ static int do_jit(struct bpf_prog *bpf_prog, int *addrs, u8 *image,
> 		u32 dst_reg = insn->dst_reg;
> 		u32 src_reg = insn->src_reg;
> 		u8 b2 = 0, b3 = 0;
> +		u8 *start_of_ldx;
> 		s64 jmp_offset;
> 		u8 jmp_cond;
> 		u8 *func;
> @@ -1278,12 +1279,30 @@ st:			if (is_imm8(insn->off))
> 		case BPF_LDX | BPF_PROBE_MEM | BPF_W:
> 		case BPF_LDX | BPF_MEM | BPF_DW:
> 		case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
> +			if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
> +				/* test src_reg, src_reg */
> +				maybe_emit_mod(&prog, src_reg, src_reg, true); /* always 1 byte */
> +				EMIT2(0x85, add_2reg(0xC0, src_reg, src_reg));
> +				/* jne start_of_ldx */
> +				EMIT2(X86_JNE, 0);
> +				/* xor dst_reg, dst_reg */
> +				emit_mov_imm32(&prog, false, dst_reg, 0);
> +				/* jmp byte_after_ldx */
> +				EMIT2(0xEB, 0);
> +
> +				/* populate jmp_offset for JNE above */
> +				temp[4] = prog - temp - 5 /* sizeof(test + jne) */;

IIUC, this case only happens for i == 1 in the loop? If so, can we use temp[5(?)] 
instead of start_of_ldx?

Thanks,
Song

> +				start_of_ldx = prog;
> +			}
> 			emit_ldx(&prog, BPF_SIZE(insn->code), dst_reg, src_reg, insn->off);
> 			if (BPF_MODE(insn->code) == BPF_PROBE_MEM) {
> 				struct exception_table_entry *ex;
> 				u8 *_insn = image + proglen;
> 				s64 delta;
> 
> +				/* populate jmp_offset for JMP above */
> +				start_of_ldx[-1] = prog - start_of_ldx;
> +
> 				if (!bpf_prog->aux->extable)
> 					break;
> 
> -- 
> 2.24.1
>