bpf: restrict stack pointer arithmetic for unprivileged

Restrict stack pointer arithmetic for unprivileged users in that
arithmetic itself must not go out of bounds as opposed to the actual
access later on. Therefore after each adjust_ptr_min_max_vals() with
a stack pointer as a destination we simulate a check_stack_access()
of 1 byte on the destination and once that fails the program is
rejected for unprivileged program loads. This is analog to map
value pointer arithmetic and needed for masking later on.
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -1387,6 +1387,31 @@ static int check_stack_read(struct bpf_verifier_env *env,
 	}
 }
 
+static int check_stack_access(struct bpf_verifier_env *env,
+			      const struct bpf_reg_state *reg,
+			      int off, int size)
+{
+	/* Stack accesses must be at a fixed offset, so that we
+	 * can determine what type of data were returned. See
+	 * check_stack_read().
+	 */
+	if (!tnum_is_const(reg->var_off)) {
+		char tn_buf[48];
+
+		tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
+		verbose(env, "variable stack access var_off=%s off=%d size=%d",
+			tn_buf, off, size);
+		return -EACCES;
+	}
+
+	if (off >= 0 || off < -MAX_BPF_STACK) {
+		verbose(env, "invalid stack off=%d size=%d\n", off, size);
+		return -EACCES;
+	}
+
+	return 0;
+}
+
 /* check read/write into map element returned by bpf_map_lookup_elem() */
 static int __check_map_access(struct bpf_verifier_env *env, u32 regno, int off,
 			      int size, bool zero_size_allowed)
@@ -1954,24 +1979,10 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
 		}
 
 	} else if (reg->type == PTR_TO_STACK) {
-		/* stack accesses must be at a fixed offset, so that we can
-		 * determine what type of data were returned.
-		 * See check_stack_read().
-		 */
-		if (!tnum_is_const(reg->var_off)) {
-			char tn_buf[48];
-
-			tnum_strn(tn_buf, sizeof(tn_buf), reg->var_off);
-			verbose(env, "variable stack access var_off=%s off=%d size=%d",
-				tn_buf, off, size);
-			return -EACCES;
-		}
 		off += reg->var_off.value;
-		if (off >= 0 || off < -MAX_BPF_STACK) {
-			verbose(env, "invalid stack off=%d size=%d\n", off,
-				size);
-			return -EACCES;
-		}
+		err = check_stack_access(env, reg, off, size);
+		if (err)
+			return err;
 
 		state = func(env, reg);
 		err = update_stack_depth(env, state, off);
@@ -3253,11 +3264,19 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
 	/* 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 && 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;
+	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;
+		}
 	}
 
 	return 0;