1 /* 2 * Copyright (C) 2015 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #ifndef _PRIVATE_WRITEPROTECTED_H 18 #define _PRIVATE_WRITEPROTECTED_H 19 20 #include <errno.h> 21 #include <string.h> 22 #include <sys/cdefs.h> 23 #include <sys/mman.h> 24 #include <sys/user.h> 25 26 #include <async_safe/log.h> 27 28 #include "private/bionic_macros.h" 29 #include "private/bionic_prctl.h" 30 31 template <typename T> 32 union WriteProtectedContents { 33 T value; 34 char padding[PAGE_SIZE]; 35 36 WriteProtectedContents() = default; 37 DISALLOW_COPY_AND_ASSIGN(WriteProtectedContents); 38 } __attribute__((aligned(PAGE_SIZE))); 39 40 // Write protected wrapper class that aligns its contents to a page boundary, 41 // and sets the memory protection to be non-writable, except when being modified 42 // explicitly. 43 template <typename T> 44 class WriteProtected { 45 static_assert(sizeof(T) < PAGE_SIZE, 46 "WriteProtected only supports contents up to PAGE_SIZE"); 47 static_assert(__is_pod(T), "WriteProtected only supports POD contents"); 48 49 WriteProtectedContents<T> contents; 50 51 public: 52 WriteProtected() = default; 53 DISALLOW_COPY_AND_ASSIGN(WriteProtected); 54 initialize()55 void initialize() { 56 // Not strictly necessary, but this will hopefully segfault if we initialize 57 // multiple times by accident. 58 memset(&contents, 0, sizeof(contents)); 59 60 if (mprotect(&contents, PAGE_SIZE, PROT_READ)) { 61 async_safe_fatal("failed to make WriteProtected nonwritable in initialize"); 62 } 63 } 64 65 const T* operator->() { 66 return &contents.value; 67 } 68 69 const T& operator*() { 70 return contents.value; 71 } 72 73 template <typename Mutator> mutate(Mutator mutator)74 void mutate(Mutator mutator) { 75 if (mprotect(&contents, PAGE_SIZE, PROT_READ | PROT_WRITE) != 0) { 76 async_safe_fatal("failed to make WriteProtected writable in mutate: %s", 77 strerror(errno)); 78 } 79 mutator(&contents.value); 80 if (mprotect(&contents, PAGE_SIZE, PROT_READ) != 0) { 81 async_safe_fatal("failed to make WriteProtected nonwritable in mutate: %s", 82 strerror(errno)); 83 } 84 } 85 }; 86 87 #endif 88