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

[Hari Bathini]

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.

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 at kernel.org>
Signed-off-by: Hari Bathini <hbathini at 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;
+
+				/*
+				 * 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;
+
+				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));
-- 
2.31.1