Heap protect (#747)

Setting configENABLE_HEAP_PROTECTOR to 1 obfuscates heap
block pointers by XORing them with an application supplied
canary value. This obfuscation helps to catch heap corruption
should a heap buffer overflow occur.

This PR also adds heap bounds checking to heap_4 and heap_5.

This PR also adds some additional integer underflow checks.

include/FreeRTOS.h

@@ -1071,6 +1071,10 @@
     #define configAPPLICATION_ALLOCATED_HEAP    0
 #endif
 
+#ifndef configENABLE_HEAP_PROTECTOR
+    #define configENABLE_HEAP_PROTECTOR    0
+#endif
+
 #ifndef configUSE_TASK_NOTIFICATIONS
     #define configUSE_TASK_NOTIFICATIONS    1
 #endif

portable/MemMang/heap_4.c

@@ -65,10 +65,13 @@
 #define heapSIZE_MAX              ( ~( ( size_t ) 0 ) )
 
 /* Check if multiplying a and b will result in overflow. */
-#define heapMULTIPLY_WILL_OVERFLOW( a, b )    ( ( ( a ) > 0 ) && ( ( b ) > ( heapSIZE_MAX / ( a ) ) ) )
+#define heapMULTIPLY_WILL_OVERFLOW( a, b )     ( ( ( a ) > 0 ) && ( ( b ) > ( heapSIZE_MAX / ( a ) ) ) )
 
 /* Check if adding a and b will result in overflow. */
-#define heapADD_WILL_OVERFLOW( a, b )         ( ( a ) > ( heapSIZE_MAX - ( b ) ) )
+#define heapADD_WILL_OVERFLOW( a, b )          ( ( a ) > ( heapSIZE_MAX - ( b ) ) )
+
+/* Check if the subtraction operation ( a - b ) will result in underflow. */
+#define heapSUBTRACT_WILL_UNDERFLOW( a, b )    ( ( a ) < ( b ) )
 
 /* MSB of the xBlockSize member of an BlockLink_t structure is used to track
  * the allocation status of a block.  When MSB of the xBlockSize member of
@@ -100,6 +103,38 @@ typedef struct A_BLOCK_LINK
     size_t xBlockSize;                     /**< The size of the free block. */
 } BlockLink_t;
 
+/* Setting configENABLE_HEAP_PROTECTOR to 1 enables heap block pointers
+ * protection using an application supplied canary value to catch heap
+ * corruption should a heap buffer overflow occur.
+ */
+#if ( configENABLE_HEAP_PROTECTOR == 1 )
+
+/**
+ * @brief Application provided function to get a random value to be used as canary.
+ *
+ * @param pxHeapCanary [out] Output parameter to return the canary value.
+ */
+    extern void vApplicationGetRandomHeapCanary( portPOINTER_SIZE_TYPE * pxHeapCanary );
+
+/* Canary value for protecting internal heap pointers. */
+    PRIVILEGED_DATA static portPOINTER_SIZE_TYPE xHeapCanary;
+
+/* Macro to load/store BlockLink_t pointers to memory. By XORing the
+ * pointers with a random canary value, heap overflows will result
+ * in randomly unpredictable pointer values which will be caught by
+ * heapVALIDATE_BLOCK_POINTER assert. */
+    #define heapPROTECT_BLOCK_POINTER( pxBlock )    ( ( BlockLink_t * ) ( ( ( portPOINTER_SIZE_TYPE ) ( pxBlock ) ) ^ xHeapCanary ) )
+#else
+
+    #define heapPROTECT_BLOCK_POINTER( pxBlock )    ( pxBlock )
+
+#endif /* configENABLE_HEAP_PROTECTOR */
+
+/* Assert that a heap block pointer is within the heap bounds. */
+#define heapVALIDATE_BLOCK_POINTER( pxBlock )                          \
+    configASSERT( ( ( uint8_t * ) ( pxBlock ) >= &( ucHeap[ 0 ] ) ) && \
+                  ( ( uint8_t * ) ( pxBlock ) <= &( ucHeap[ configTOTAL_HEAP_SIZE - 1 ] ) ) )
+
 /*-----------------------------------------------------------*/
 
 /*
@@ -206,12 +241,14 @@ void * pvPortMalloc( size_t xWantedSize )
                 /* Traverse the list from the start (lowest address) block until
                  * one of adequate size is found. */
                 pxPreviousBlock = &xStart;
-                pxBlock = xStart.pxNextFreeBlock;
+                pxBlock = heapPROTECT_BLOCK_POINTER( xStart.pxNextFreeBlock );
+                heapVALIDATE_BLOCK_POINTER( pxBlock );
 
-                while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != NULL ) )
+                while( ( pxBlock->xBlockSize < xWantedSize ) && ( pxBlock->pxNextFreeBlock != heapPROTECT_BLOCK_POINTER( NULL ) ) )
                 {
                     pxPreviousBlock = pxBlock;
-                    pxBlock = pxBlock->pxNextFreeBlock;
+                    pxBlock = heapPROTECT_BLOCK_POINTER( pxBlock->pxNextFreeBlock );
+                    heapVALIDATE_BLOCK_POINTER( pxBlock );
                 }
 
                 /* If the end marker was reached then a block of adequate size
@@ -220,7 +257,8 @@ void * pvPortMalloc( size_t xWantedSize )
                 {
                     /* Return the memory space pointed to - jumping over the
                      * BlockLink_t structure at its start. */
-                    pvReturn = ( void * ) ( ( ( uint8_t * ) pxPreviousBlock->pxNextFreeBlock ) + xHeapStructSize );
+                    pvReturn = ( void * ) ( ( ( uint8_t * ) heapPROTECT_BLOCK_POINTER( pxPreviousBlock->pxNextFreeBlock ) ) + xHeapStructSize );
+                    heapVALIDATE_BLOCK_POINTER( pvReturn );
 
                     /* This block is being returned for use so must be taken out
                      * of the list of free blocks. */
@@ -228,6 +266,8 @@ void * pvPortMalloc( size_t xWantedSize )
 
                     /* If the block is larger than required it can be split into
                      * two. */
+                    configASSERT( heapSUBTRACT_WILL_UNDERFLOW( pxBlock->xBlockSize, xWantedSize ) == 0 );
+
                     if( ( pxBlock->xBlockSize - xWantedSize ) > heapMINIMUM_BLOCK_SIZE )
                     {
                         /* This block is to be split into two.  Create a new
@@ -244,7 +284,7 @@ void * pvPortMalloc( size_t xWantedSize )
 
                         /* Insert the new block into the list of free blocks. */
                         pxNewBlockLink->pxNextFreeBlock = pxPreviousBlock->pxNextFreeBlock;
-                        pxPreviousBlock->pxNextFreeBlock = pxNewBlockLink;
+                        pxPreviousBlock->pxNextFreeBlock = heapPROTECT_BLOCK_POINTER( pxNewBlockLink );
                     }
                     else
                     {
@@ -319,6 +359,7 @@ void vPortFree( void * pv )
         /* This casting is to keep the compiler from issuing warnings. */
         pxLink = ( void * ) puc;
 
+        heapVALIDATE_BLOCK_POINTER( pxLink );
         configASSERT( heapBLOCK_IS_ALLOCATED( pxLink ) != 0 );
         configASSERT( pxLink->pxNextFreeBlock == NULL );
 
@@ -331,7 +372,12 @@ void vPortFree( void * pv )
                 heapFREE_BLOCK( pxLink );
                 #if ( configHEAP_CLEAR_MEMORY_ON_FREE == 1 )
                 {
-                    ( void ) memset( puc + xHeapStructSize, 0, pxLink->xBlockSize - xHeapStructSize );
+                    /* Check for underflow as this can occur if xBlockSize is
+                     * overwritten in a heap block. */
+                    if( heapSUBTRACT_WILL_UNDERFLOW( pxLink->xBlockSize, xHeapStructSize ) == 0 )
+                    {
+                        ( void ) memset( puc + xHeapStructSize, 0, pxLink->xBlockSize - xHeapStructSize );
+                    }
                 }
                 #endif
 
@@ -414,9 +460,15 @@ static void prvHeapInit( void ) /* PRIVILEGED_FUNCTION */
 
     pucAlignedHeap = ( uint8_t * ) uxAddress;
 
+    #if ( configENABLE_HEAP_PROTECTOR == 1 )
+    {
+        vApplicationGetRandomHeapCanary( &( xHeapCanary ) );
+    }
+    #endif
+
     /* xStart is used to hold a pointer to the first item in the list of free
      * blocks.  The void cast is used to prevent compiler warnings. */
-    xStart.pxNextFreeBlock = ( void * ) pucAlignedHeap;
+    xStart.pxNextFreeBlock = ( void * ) heapPROTECT_BLOCK_POINTER( pucAlignedHeap );
     xStart.xBlockSize = ( size_t ) 0;
 
     /* pxEnd is used to mark the end of the list of free blocks and is inserted
@@ -426,13 +478,13 @@ static void prvHeapInit( void ) /* PRIVILEGED_FUNCTION */
     uxAddress &= ~( ( portPOINTER_SIZE_TYPE ) portBYTE_ALIGNMENT_MASK );
     pxEnd = ( BlockLink_t * ) uxAddress;
     pxEnd->xBlockSize = 0;
-    pxEnd->pxNextFreeBlock = NULL;
+    pxEnd->pxNextFreeBlock = heapPROTECT_BLOCK_POINTER( NULL );
 
     /* To start with there is a single free block that is sized to take up the
      * entire heap space, minus the space taken by pxEnd. */
     pxFirstFreeBlock = ( BlockLink_t * ) pucAlignedHeap;
     pxFirstFreeBlock->xBlockSize = ( size_t ) ( uxAddress - ( portPOINTER_SIZE_TYPE ) pxFirstFreeBlock );
-    pxFirstFreeBlock->pxNextFreeBlock = pxEnd;
+    pxFirstFreeBlock->pxNextFreeBlock = heapPROTECT_BLOCK_POINTER( pxEnd );
 
     /* Only one block exists - and it covers the entire usable heap space. */
     xMinimumEverFreeBytesRemaining = pxFirstFreeBlock->xBlockSize;
@@ -447,11 +499,16 @@ static void prvInsertBlockIntoFreeList( BlockLink_t * pxBlockToInsert ) /* PRIVI
 
     /* Iterate through the list until a block is found that has a higher address
      * than the block being inserted. */
-    for( pxIterator = &xStart; pxIterator->pxNextFreeBlock < pxBlockToInsert; pxIterator = pxIterator->pxNextFreeBlock )
+    for( pxIterator = &xStart; heapPROTECT_BLOCK_POINTER( pxIterator->pxNextFreeBlock ) < pxBlockToInsert; pxIterator = heapPROTECT_BLOCK_POINTER( pxIterator->pxNextFreeBlock ) )
     {
         /* Nothing to do here, just iterate to the right position. */
     }
 
+    if( pxIterator != &xStart )
+    {
+        heapVALIDATE_BLOCK_POINTER( pxIterator );
+    }
+
     /* Do the block being inserted, and the block it is being inserted after
      * make a contiguous block of memory? */
     puc = ( uint8_t * ) pxIterator;
@@ -470,17 +527,17 @@ static void prvInsertBlockIntoFreeList( BlockLink_t * pxBlockToInsert ) /* PRIVI
      * make a contiguous block of memory? */
     puc = ( uint8_t * ) pxBlockToInsert;
 
-    if( ( puc + pxBlockToInsert->xBlockSize ) == ( uint8_t * ) pxIterator->pxNextFreeBlock )
+    if( ( puc + pxBlockToInsert->xBlockSize ) == ( uint8_t * ) heapPROTECT_BLOCK_POINTER( pxIterator->pxNextFreeBlock ) )
     {
-        if( pxIterator->pxNextFreeBlock != pxEnd )
+        if( heapPROTECT_BLOCK_POINTER( pxIterator->pxNextFreeBlock ) != pxEnd )
         {
             /* Form one big block from the two blocks. */
-            pxBlockToInsert->xBlockSize += pxIterator->pxNextFreeBlock->xBlockSize;
+            pxBlockToInsert->xBlockSize += heapPROTECT_BLOCK_POINTER( pxIterator->pxNextFreeBlock )->xBlockSize;
-            pxBlockToInsert->pxNextFreeBlock = pxIterator->pxNextFreeBlock->pxNextFreeBlock;
+            pxBlockToInsert->pxNextFreeBlock = heapPROTECT_BLOCK_POINTER( pxIterator->pxNextFreeBlock )->pxNextFreeBlock;
         }
         else
         {
-            pxBlockToInsert->pxNextFreeBlock = pxEnd;
+            pxBlockToInsert->pxNextFreeBlock = heapPROTECT_BLOCK_POINTER( pxEnd );
         }
     }
     else
@@ -494,7 +551,7 @@ static void prvInsertBlockIntoFreeList( BlockLink_t * pxBlockToInsert ) /* PRIVI
      * to itself. */
     if( pxIterator != pxBlockToInsert )
     {
-        pxIterator->pxNextFreeBlock = pxBlockToInsert;
+        pxIterator->pxNextFreeBlock = heapPROTECT_BLOCK_POINTER( pxBlockToInsert );
     }
     else
     {
@@ -510,7 +567,7 @@ void vPortGetHeapStats( HeapStats_t * pxHeapStats )
 
     vTaskSuspendAll();
     {
-        pxBlock = xStart.pxNextFreeBlock;
+        pxBlock = heapPROTECT_BLOCK_POINTER( xStart.pxNextFreeBlock );
 
         /* pxBlock will be NULL if the heap has not been initialised.  The heap
          * is initialised automatically when the first allocation is made. */
@@ -534,7 +591,7 @@ void vPortGetHeapStats( HeapStats_t * pxHeapStats )
 
                 /* Move to the next block in the chain until the last block is
                  * reached. */
-                pxBlock = pxBlock->pxNextFreeBlock;
+                pxBlock = heapPROTECT_BLOCK_POINTER( pxBlock->pxNextFreeBlock );
             }
         }
     }