Re: [PATCH v2 8/8] bpf ppc32: Add addr > TASK_SIZE_MAX explicit check

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 17/09/2021 à 17:30, Hari Bathini a écrit :
> With KUAP enabled, any kernel code which wants to access userspace
> needs to be surrounded by disable-enable KUAP. But that is not
> happening for BPF_PROBE_MEM load instruction. Though PPC32 does not
> support read protection, considering the fact that PTR_TO_BTF_ID
> (which uses BPF_PROBE_MEM mode) could either be a valid kernel pointer
> or NULL but should never be a pointer to userspace address, execute
> BPF_PROBE_MEM load only if addr > TASK_SIZE_MAX, otherwise set
> dst_reg=0 and move on.

Same comment as patch 6.

> 
> This will catch NULL, valid or invalid userspace pointers. Only bad
> kernel pointer will be handled by BPF exception table.
> 
> [Alexei suggested for x86]
> Suggested-by: Alexei Starovoitov <ast@kernel.org>
> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
> ---
> 
> Changes in v2:
> * New patch to handle bad userspace pointers on PPC32.
> 
> 
>   arch/powerpc/net/bpf_jit_comp32.c | 39 +++++++++++++++++++++++++++++++
>   1 file changed, 39 insertions(+)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index c6262289dcc4..faa8047fcf4a 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -821,6 +821,45 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   		/* dst = *(u64 *)(ul) (src + off) */
>   		case BPF_LDX | BPF_MEM | BPF_DW:
>   		case BPF_LDX | BPF_PROBE_MEM | BPF_DW:
> +			/*
> +			 * As PTR_TO_BTF_ID that uses BPF_PROBE_MEM mode could either be a valid
> +			 * kernel pointer or NULL but not a userspace address, execute BPF_PROBE_MEM
> +			 * load only if addr > TASK_SIZE_MAX, otherwise set dst_reg=0 and move on.
> +			 */
> +			if (BPF_MODE(code) == BPF_PROBE_MEM) {
> +				bool extra_insn_needed = false;
> +				unsigned int adjusted_idx;
> +
> +				/*
> +				 * For BPF_DW case, "li reg_h,0" would be needed when
> +				 * !fp->aux->verifier_zext. Adjust conditional branch'ing
> +				 * address accordingly.
> +				 */
> +				if ((size == BPF_DW) && !fp->aux->verifier_zext)
> +					extra_insn_needed = true;

Don't make it too complicated. That's a fallback that should never 
happen, no need to optimise. You can put that instruction all the time 
(or put a NOP) and keep the jumps always the same.

> +
> +				/*
> +				 * Need to jump two instructions instead of one for BPF_DW case
> +				 * as there are two load instructions for dst_reg_h & dst_reg
> +				 * respectively.
> +				 */
> +				adjusted_idx = (size == BPF_DW) ? 1 : 0;

Same comment as patch 6, drop adjusted_idx and do an if/else directly 
for the PPC_JMP.

> +
> +				EMIT(PPC_RAW_ADDI(b2p[TMP_REG], src_reg, off));
> +				PPC_LI32(_R0, TASK_SIZE_MAX);
> +				EMIT(PPC_RAW_CMPLW(b2p[TMP_REG], _R0));
> +				PPC_BCC(COND_GT, (ctx->idx + 4 + (extra_insn_needed ? 1 : 0)) * 4);
> +				EMIT(PPC_RAW_LI(dst_reg, 0));
> +				/*
> +				 * Note that "li reg_h,0" is emitted for BPF_B/H/W case,
> +				 * if necessary. So, jump there insted of emitting an
> +				 * additional "li reg_h,0" instruction.
> +				 */
> +				if (extra_insn_needed)
> +					EMIT(PPC_RAW_LI(dst_reg_h, 0));
> +				PPC_JMP((ctx->idx + 2 + adjusted_idx) * 4);
> +			}
> +
>   			switch (size) {
>   			case BPF_B:
>   				EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));
>