bpf: Refactor and streamline bounds check into helper

mainline inclusion
from mainline-v5.12-rc8
commit 073815b7
category: bugfix
bugzilla: NA
CVE: CVE-2021-29155

--------------------------------

Move the bounds check in adjust_ptr_min_max_vals() into a small helper named
sanitize_check_bounds() in order to simplify the former a bit.
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Reviewed-by: NJohn Fastabend <john.fastabend@gmail.com>
Acked-by: NAlexei Starovoitov <ast@kernel.org>
Conflicts:
  kernel/bpf/verifier.c
[yyl: bypass_spec_v1 is not introduced in kernel-4.19,
 use allow_ptr_leaks instead]
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>
Reviewed-by: NKuohai Xu <xukuohai@huawei.com>
Reviewed-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>

kernel/bpf/verifier.c

@@ -2911,6 +2911,72 @@ static int sanitize_err(struct bpf_verifier_env *env,
 	return -EACCES;
 }
 
+/* check that stack access falls within stack limits and that 'reg' doesn't
+ * have a variable offset.
+ *
+ * Variable offset is prohibited for unprivileged mode for simplicity since it
+ * requires corresponding support in Spectre masking for stack ALU.  See also
+ * retrieve_ptr_limit().
+ *
+ *
+ * 'off' includes 'reg->off'.
+ */
+static int check_stack_access_for_ptr_arithmetic(
+				struct bpf_verifier_env *env,
+				int regno,
+				const struct bpf_reg_state *reg,
+				int off)
+{
+	if (!tnum_is_const(reg->var_off)) {
+		char tn_buf[48];
+
+		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+		verbose(env, "R%d variable stack access prohibited for !root, var_off=%s off=%d\n",
+			regno, tn_buf, off);
+		return -EACCES;
+	}
+
+	if (off >= 0 || off < -MAX_BPF_STACK) {
+		verbose(env, "R%d stack pointer arithmetic goes out of range, "
+			"prohibited for !root; off=%d\n", regno, off);
+		return -EACCES;
+	}
+
+	return 0;
+}
+
+static int sanitize_check_bounds(struct bpf_verifier_env *env,
+				 const struct bpf_insn *insn,
+				 const struct bpf_reg_state *dst_reg)
+{
+	u32 dst = insn->dst_reg;
+
+	/* For unprivileged we require that resulting offset must be in bounds
+	 * in order to be able to sanitize access later on.
+	 */
+	if (env->allow_ptr_leaks)
+		return 0;
+
+	switch (dst_reg->type) {
+	case PTR_TO_STACK:
+		if (check_stack_access_for_ptr_arithmetic(env, dst, dst_reg,
+					dst_reg->off + dst_reg->var_off.value))
+			return -EACCES;
+		break;
+	case PTR_TO_MAP_VALUE:
+		if (check_map_access(env, dst, dst_reg->off, 1, false)) {
+			verbose(env, "R%d pointer arithmetic of map value goes out of range, "
+				"prohibited for !root\n", dst);
+			return -EACCES;
+		}
+		break;
+	default:
+		break;
+	}
+
+	return 0;
+}
+
 /* Handles arithmetic on a pointer and a scalar: computes new min/max and var_off.
  * Caller should also handle BPF_MOV case separately.
  * If we return -EACCES, caller may want to try again treating pointer as a
@@ -3118,24 +3184,9 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	__reg_deduce_bounds(dst_reg);
 	__reg_bound_offset(dst_reg);
 
-	/* For unprivileged we require that resulting offset must be in bounds
-	 * in order to be able to sanitize access later on.
-	 */
-	if (!env->allow_ptr_leaks) {
-		if (dst_reg->type == PTR_TO_MAP_VALUE &&
-		    check_map_access(env, dst, dst_reg->off, 1, false)) {
-			verbose(env, "R%d pointer arithmetic of map value goes out of range, "
-				"prohibited for !root\n", dst);
-			return -EACCES;
-		} else if (dst_reg->type == PTR_TO_STACK &&
-			   check_stack_access(env, dst_reg, dst_reg->off +
-					      dst_reg->var_off.value, 1)) {
-			verbose(env, "R%d stack pointer arithmetic goes out of range, "
-				"prohibited for !root\n", dst);
-			return -EACCES;
-		}
-	}
 
+	if (sanitize_check_bounds(env, insn, dst_reg) < 0)
+		return -EACCES;
 	return 0;
 }