Re: [PATCH v2 6/8] bpf ppc64: Add addr > TASK_SIZE_MAX explicit check
From: Christophe Leroy <hidden>
Date: 2021-09-17 16:50:42
Also in:
bpf, linuxppc-dev
Le 17/09/2021 à 17:30, Hari Bathini a écrit :
From: Ravi Bangoria <redacted> On PPC64 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. So, when BPF program tries to access invalid userspace address, page-fault handler considers it as bad KUAP fault: Kernel attempted to read user page (d0000000) - exploit attempt? (uid: 0) 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.
You should do like copy_from_kernel_nofault_allowed() and use the same criterias as is_kernel_addr() instead of using TASK_SIZE_MAX.
quoted hunk ↗ jump to hunk
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: Ravi Bangoria <redacted> Signed-off-by: Hari Bathini <hbathini@linux.ibm.com> --- Changes in v2: * Refactored the code based on Christophe's comments. arch/powerpc/net/bpf_jit_comp64.c | 23 +++++++++++++++++++++++ 1 file changed, 23 insertions(+)diff --git a/arch/powerpc/net/bpf_jit_comp64.c b/arch/powerpc/net/bpf_jit_comp64.c index 2fc10995f243..eb28dbc67151 100644 --- a/arch/powerpc/net/bpf_jit_comp64.c +++ b/arch/powerpc/net/bpf_jit_comp64.c@@ -769,6 +769,29 @@ 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) { + unsigned int adjusted_idx; + + /* + * Check if 'off' is word aligned because PPC_BPF_LL() + * (BPF_DW case) generates two instructions if 'off' is not + * word-aligned and one instruction otherwise. + */ + adjusted_idx = ((BPF_SIZE(code) == BPF_DW) && (off & 3)) ? 1 : 0;
No need of ( ) around 'BPF_SIZE(code) == BPF_DW'
+ + EMIT(PPC_RAW_ADDI(b2p[TMP_REG_1], src_reg, off)); + PPC_LI64(b2p[TMP_REG_2], TASK_SIZE_MAX); + EMIT(PPC_RAW_CMPLD(b2p[TMP_REG_1], b2p[TMP_REG_2])); + PPC_BCC(COND_GT, (ctx->idx + 4) * 4); + EMIT(PPC_RAW_LI(dst_reg, 0)); + PPC_JMP((ctx->idx + 2 + adjusted_idx) * 4);
I think it would be more explicit if you drop adjusted_idx and do : if (BPF_SIZE(code) == BPF_DW) && (off & 3) 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));