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Message-ID: <aa3db398-5d44-c68c-6f74-027e31521177@csgroup.eu> Date: Thu, 14 Sep 2023 06:18:09 +0000 From: Christophe Leroy <christophe.leroy@...roup.eu> To: Hari Bathini <hbathini@...ux.ibm.com>, "naveen.n.rao@...ux.ibm.com" <naveen.n.rao@...ux.ibm.com>, "mpe@...erman.id.au" <mpe@...erman.id.au>, "ast@...nel.org" <ast@...nel.org>, "daniel@...earbox.net" <daniel@...earbox.net> CC: "paulus@...ba.org" <paulus@...ba.org>, "andrii@...nel.org" <andrii@...nel.org>, "kafai@...com" <kafai@...com>, "songliubraving@...com" <songliubraving@...com>, "yhs@...com" <yhs@...com>, "john.fastabend@...il.com" <john.fastabend@...il.com>, "kpsingh@...nel.org" <kpsingh@...nel.org>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>, "bpf@...r.kernel.org" <bpf@...r.kernel.org>, "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org> Subject: Re: [PATCH v4 8/8] bpf ppc32: Access only if addr is kernel address 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@...nel.org> > Signed-off-by: Hari Bathini <hbathini@...ux.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));
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