Re: [PATCH v4 8/8] bpf ppc32: Access only if addr is kernel address

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

Hi,

Le 29/09/2021 à 13:18, 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 is kernel address, otherwise set
> dst_reg=0 and move on.

While looking at the series "bpf: verifier: stop emitting zext for LDX" 
from Puranjay I got a question on this old commit, see below.

> 
> 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 v4:
> * Adjusted the emit code to avoid using temporary reg.
> 
> 
>   arch/powerpc/net/bpf_jit_comp32.c | 34 +++++++++++++++++++++++++++++++
>   1 file changed, 34 insertions(+)
> 
> diff --git a/arch/powerpc/net/bpf_jit_comp32.c b/arch/powerpc/net/bpf_jit_comp32.c
> index 6ee13a09c70d..2ac81563c78d 100644
> --- a/arch/powerpc/net/bpf_jit_comp32.c
> +++ b/arch/powerpc/net/bpf_jit_comp32.c
> @@ -818,6 +818,40 @@ int bpf_jit_build_body(struct bpf_prog *fp, u32 *image, struct codegen_context *
>   		case BPF_LDX | BPF_PROBE_MEM | BPF_W:
>   		case BPF_LDX | BPF_MEM | BPF_DW: /* dst = *(u64 *)(ul) (src + off) */
>   		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 is kernel address (see is_kernel_addr()), otherwise
> +			 * set dst_reg=0 and move on.
> +			 */
> +			if (BPF_MODE(code) == BPF_PROBE_MEM) {
> +				PPC_LI32(_R0, TASK_SIZE - off);
> +				EMIT(PPC_RAW_CMPLW(src_reg, _R0));
> +				PPC_BCC(COND_GT, (ctx->idx + 5) * 4);
> +				EMIT(PPC_RAW_LI(dst_reg, 0));
> +				/*
> +				 * For BPF_DW case, "li reg_h,0" would be needed when
> +				 * !fp->aux->verifier_zext. Emit NOP otherwise.
> +				 *
> +				 * 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 (size == BPF_DW && !fp->aux->verifier_zext)
> +					EMIT(PPC_RAW_LI(dst_reg_h, 0));
> +				else
> +					EMIT(PPC_RAW_NOP());

While do you need a NOP in the else case ? Can't we just emit no 
instruction in that case ?


> +				/*
> +				 * 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.
> +				 */
> +				if (size == BPF_DW)
> +					PPC_JMP((ctx->idx + 3) * 4);
> +				else
> +					PPC_JMP((ctx->idx + 2) * 4);
> +			}
> +
>   			switch (size) {
>   			case BPF_B:
>   				EMIT(PPC_RAW_LBZ(dst_reg, src_reg, off));