selftests/bpf: Test unbounded var_off stack access

Test the case when reg->smax_value is too small/big and can overflow,
and separately min and max values outside of stack bounds.

Example of output:
  # ./test_verifier
  #856/p indirect variable-offset stack access, unbounded OK
  #857/p indirect variable-offset stack access, max out of bound OK
  #858/p indirect variable-offset stack access, min out of bound OK
Signed-off-by: NAndrey Ignatov <rdna@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/verifier/var_off.c

@@ -40,7 +40,35 @@
 	.prog_type = BPF_PROG_TYPE_LWT_IN,
 },
 {
-	"indirect variable-offset stack access, out of bound",
+	"indirect variable-offset stack access, unbounded",
+	.insns = {
+	BPF_MOV64_IMM(BPF_REG_2, 6),
+	BPF_MOV64_IMM(BPF_REG_3, 28),
+	/* Fill the top 16 bytes of the stack. */
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -16, 0),
+	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
+	/* Get an unknown value. */
+	BPF_LDX_MEM(BPF_DW, BPF_REG_4, BPF_REG_1, offsetof(struct bpf_sock_ops,
+							   bytes_received)),
+	/* Check the lower bound but don't check the upper one. */
+	BPF_JMP_IMM(BPF_JSLT, BPF_REG_4, 0, 4),
+	/* Point the lower bound to initialized stack. Offset is now in range
+	 * from fp-16 to fp+0x7fffffffffffffef, i.e. max value is unbounded.
+	 */
+	BPF_ALU64_IMM(BPF_SUB, BPF_REG_4, 16),
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_4, BPF_REG_10),
+	BPF_MOV64_IMM(BPF_REG_5, 8),
+	/* Dereference it indirectly. */
+	BPF_EMIT_CALL(BPF_FUNC_getsockopt),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.errstr = "R4 unbounded indirect variable offset stack access",
+	.result = REJECT,
+	.prog_type = BPF_PROG_TYPE_SOCK_OPS,
+},
+{
+	"indirect variable-offset stack access, max out of bound",
 	.insns = {
 	/* Fill the top 8 bytes of the stack */
 	BPF_ST_MEM(BPF_DW, BPF_REG_10, -8, 0),
@@ -60,7 +88,32 @@
 	BPF_EXIT_INSN(),
 	},
 	.fixup_map_hash_8b = { 5 },
-	.errstr = "invalid stack type R2 var_off",
+	.errstr = "R2 max value is outside of stack bound",
+	.result = REJECT,
+	.prog_type = BPF_PROG_TYPE_LWT_IN,
+},
+{
+	"indirect variable-offset stack access, min out of bound",
+	.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, 516),
+	/* add it to fp.  We now have either fp-516 or fp-512, but
+	 * we don't know which
+	 */
+	BPF_ALU64_REG(BPF_ADD, BPF_REG_2, BPF_REG_10),
+	/* dereference it indirectly */
+	BPF_LD_MAP_FD(BPF_REG_1, 0),
+	BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem),
+	BPF_MOV64_IMM(BPF_REG_0, 0),
+	BPF_EXIT_INSN(),
+	},
+	.fixup_map_hash_8b = { 5 },
+	.errstr = "R2 min value is outside of stack bound",
 	.result = REJECT,
 	.prog_type = BPF_PROG_TYPE_LWT_IN,
 },