selftests/bpf: variable offset negative tests

Variable ctx accesses and stack accesses aren't allowed, because we can't
 determine what type of value will be read.
Signed-off-by: NEdward Cree <ecree@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

tools/testing/selftests/bpf/test_verifier.c

@@ -5980,6 +5980,47 @@ static struct bpf_test tests[] = {
 		.errstr = "R0 min value is negative, either use unsigned index or do a if (index >=0) check.",
 		.result = REJECT,
 	},
+	{
+		"variable-offset ctx access",
+		.insns = {
+			/* Get an unknown value */
+			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+			/* Make it small and 4-byte aligned */
+			BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4),
+			/* add it to skb.  We now have either &skb->len or
+			 * &skb->pkt_type, but we don't know which
+			 */
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_1, BPF_REG_2),
+			/* dereference it */
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_1, 0),
+			BPF_EXIT_INSN(),
+		},
+		.errstr = "variable ctx access var_off=(0x0; 0x4)",
+		.result = REJECT,
+		.prog_type = BPF_PROG_TYPE_LWT_IN,
+	},
+	{
+		"variable-offset stack access",
+		.insns = {
+			/* Fill the top 8 bytes of the stack */
+			BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+			/* Get an unknown value */
+			BPF_LDX_MEM(BPF_W, BPF_REG_2, BPF_REG_1, 0),
+			/* Make it small and 4-byte aligned */
+			BPF_ALU64_IMM(BPF_AND, BPF_REG_2, 4),
+			BPF_ALU64_IMM(BPF_SUB, BPF_REG_2, 8),
+			/* add it to fp.  We now have either fp-4 or fp-8, but
+			 * we don't know which
+			 */
+			BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+			/* dereference it */
+			BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF_REG_2, 0),
+			BPF_EXIT_INSN(),
+		},
+		.errstr = "variable stack access var_off=(0xfffffffffffffff8; 0x4)",
+		.result = REJECT,
+		.prog_type = BPF_PROG_TYPE_LWT_IN,
+	},
 };
 
 static int probe_filter_length(const struct bpf_insn *fp)