diff --git a/components/esp32/ld/esp32.rom.ld b/components/esp32/ld/esp32.rom.ld
index ccd9851a5..6058a9e61 100644
--- a/components/esp32/ld/esp32.rom.ld
+++ b/components/esp32/ld/esp32.rom.ld
@@ -34,7 +34,6 @@ PROVIDE ( cache_sram_mmu_set_rom = 0x400097f4 );
 /* This is static function, but can be used, not generated by script*/
 PROVIDE ( calc_rtc_memory_crc = 0x40008170 );
 PROVIDE ( calloc = 0x4000bee4 );
-PROVIDE ( _calloc_r = 0x4000bbf8 );
 PROVIDE ( __clear_cache = 0x40063860 );
 PROVIDE ( _close_r = 0x4000bd3c );
 PROVIDE ( co_default_bdaddr = 0x3ffae704 );
diff --git a/components/heap/heap_caps.c b/components/heap/heap_caps.c
index ad971c176..4f9c016a5 100644
--- a/components/heap/heap_caps.c
+++ b/components/heap/heap_caps.c
@@ -33,26 +33,21 @@ possible. This should optimize the amount of RAM accessible to the code without
 
 /*
   This takes a memory chunk in a region that can be addressed as both DRAM as well as IRAM. It will convert it to
-  IRAM in such a way that it can be later freed. It assumes both the address as wel as the length to be word-aligned.
+  IRAM in such a way that it can be later freed. It assumes both the address as well as the length to be word-aligned.
   It returns a region that's 1 word smaller than the region given because it stores the original Dram address there.
-
-  In theory, we can also make this work by prepending a struct that looks similar to the block link struct used by the
-  heap allocator itself, which will allow inspection tools relying on any block returned from any sort of malloc to
-  have such a block in front of it, work. We may do this later, if/when there is demand for it. For now, a simple
-  pointer is used.
 */
 IRAM_ATTR static void *dram_alloc_to_iram_addr(void *addr, size_t len)
 {
-    uint32_t dstart = (int)addr; //First word
-    uint32_t dend = ((int)addr) + len - 4; //Last word
-    assert(dstart >= SOC_DIRAM_DRAM_LOW);
-    assert(dend <= SOC_DIRAM_DRAM_HIGH);
+    uintptr_t dstart = (uintptr_t)addr; //First word
+    uintptr_t dend = dstart + len - 4; //Last word
+    assert(esp_ptr_in_diram_dram((void *)dstart));
+    assert(esp_ptr_in_diram_dram((void *)dend));
     assert((dstart & 3) == 0);
     assert((dend & 3) == 0);
     uint32_t istart = SOC_DIRAM_IRAM_LOW + (SOC_DIRAM_DRAM_HIGH - dend);
     uint32_t *iptr = (uint32_t *)istart;
     *iptr = dstart;
-    return (void *)(iptr + 1);
+    return iptr + 1;
 }
 
 bool heap_caps_match(const heap_t *heap, uint32_t caps)
@@ -67,6 +62,12 @@ IRAM_ATTR void *heap_caps_malloc( size_t size, uint32_t caps )
 {
     void *ret = NULL;
 
+    if (size > HEAP_SIZE_MAX) {
+        // Avoids int overflow when adding small numbers to size, or
+        // calculating 'end' from start+size, by limiting 'size' to the possible range
+        return NULL;
+    }
+
     if (caps & MALLOC_CAP_EXEC) {
         //MALLOC_CAP_EXEC forces an alloc from IRAM. There is a region which has both this as well as the following
         //caps, but the following caps are not possible for IRAM.  Thus, the combination is impossible and we return
@@ -75,9 +76,17 @@ IRAM_ATTR void *heap_caps_malloc( size_t size, uint32_t caps )
         if ((caps & MALLOC_CAP_8BIT) || (caps & MALLOC_CAP_DMA)) {
             return NULL;
         }
-        //If any, EXEC memory should be 32-bit aligned, so round up to the next multiple of 4.
-        size = (size + 3) & (~3);
+
+        caps |= MALLOC_CAP_32BIT; // IRAM is 32-bit accessible RAM
     }
+
+    if (caps & MALLOC_CAP_32BIT) {
+        /* 32-bit accessible RAM should allocated in 4 byte aligned sizes
+         * (Future versions of ESP-IDF should possibly fail if an invalid size is requested)
+         */
+        size = (size + 3) & (~3); // int overflow checked above
+    }
+
     for (int prio = 0; prio < SOC_MEMORY_TYPE_NO_PRIOS; prio++) {
         //Iterate over heaps and check capabilities at this priority
         heap_t *heap;
@@ -90,13 +99,13 @@ IRAM_ATTR void *heap_caps_malloc( size_t size, uint32_t caps )
                 //doesn't cover, see if they're available in other prios.
                 if ((get_all_caps(heap) & caps) == caps) {
                     //This heap can satisfy all the requested capabilities. See if we can grab some memory using it.
-                    if ((caps & MALLOC_CAP_EXEC) && heap->start >= SOC_DIRAM_DRAM_LOW && heap->start < SOC_DIRAM_DRAM_HIGH) {
+                    if ((caps & MALLOC_CAP_EXEC) && esp_ptr_in_diram_dram((void *)heap->start)) {
                         //This is special, insofar that what we're going to get back is a DRAM address. If so,
                         //we need to 'invert' it (lowest address in DRAM == highest address in IRAM and vice-versa) and
                         //add a pointer to the DRAM equivalent before the address we're going to return.
-                        ret = multi_heap_malloc(heap->heap, size + 4);
+                        ret = multi_heap_malloc(heap->heap, size + 4);  // int overflow checked above
                         if (ret != NULL) {
-                            return dram_alloc_to_iram_addr(ret, size + 4);
+                            return dram_alloc_to_iram_addr(ret, size + 4);  // int overflow checked above
                         }
                     } else {
                         //Just try to alloc, nothing special.
@@ -243,13 +252,11 @@ IRAM_ATTR static heap_t *find_containing_heap(void *ptr )
 
 IRAM_ATTR void heap_caps_free( void *ptr)
 {
-    intptr_t p = (intptr_t)ptr;
-
     if (ptr == NULL) {
         return;
     }
 
-    if ((p >= SOC_DIRAM_IRAM_LOW) && (p <= SOC_DIRAM_IRAM_HIGH)) {
+    if (esp_ptr_in_diram_iram(ptr)) {
         //Memory allocated here is actually allocated in the DRAM alias region and
         //cannot be de-allocated as usual. dram_alloc_to_iram_addr stores a pointer to
         //the equivalent DRAM address, though; free that.
@@ -273,6 +280,10 @@ IRAM_ATTR void *heap_caps_realloc( void *ptr, size_t size, int caps)
         return NULL;
     }
 
+    if (size > HEAP_SIZE_MAX) {
+        return NULL;
+    }
+
     heap_t *heap = find_containing_heap(ptr);
 
     assert(heap != NULL && "realloc() pointer is outside heap areas");
@@ -305,12 +316,18 @@ IRAM_ATTR void *heap_caps_realloc( void *ptr, size_t size, int caps)
 
 IRAM_ATTR void *heap_caps_calloc( size_t n, size_t size, uint32_t caps)
 {
-    void *r;
-    r = heap_caps_malloc(n*size, caps);
-    if (r != NULL) {
-        bzero(r, n*size);
+    void *result;
+    size_t size_bytes;
+
+    if (__builtin_mul_overflow(n, size, &size_bytes)) {
+        return NULL;
     }
-    return r;
+
+    result = heap_caps_malloc(size_bytes, caps);
+    if (result != NULL) {
+        bzero(result, size_bytes);
+    }
+    return result;
 }
 
 size_t heap_caps_get_free_size( uint32_t caps )
diff --git a/components/heap/heap_caps_init.c b/components/heap/heap_caps_init.c
index a2b66bfc7..65eb452d2 100644
--- a/components/heap/heap_caps_init.c
+++ b/components/heap/heap_caps_init.c
@@ -31,7 +31,9 @@ struct registered_heap_ll registered_heaps;
 
 static void register_heap(heap_t *region)
 {
-    region->heap = multi_heap_register((void *)region->start, region->end - region->start);
+    size_t heap_size = region->end - region->start;
+    assert(heap_size <= HEAP_SIZE_MAX);
+    region->heap = multi_heap_register((void *)region->start, heap_size);
     if (region->heap != NULL) {
         ESP_EARLY_LOGD(TAG, "New heap initialised at %p", region->heap);
     }
diff --git a/components/heap/heap_private.h b/components/heap/heap_private.h
index a8a0ac9fd..f5a61f327 100644
--- a/components/heap/heap_private.h
+++ b/components/heap/heap_private.h
@@ -28,6 +28,8 @@ extern "C" {
    for heap_caps_init.c to share heap information with heap_caps.c
 */
 
+#define HEAP_SIZE_MAX (SOC_MAX_CONTIGUOUS_RAM_SIZE)
+
 /* Type for describing each registered heap */
 typedef struct heap_t_ {
     uint32_t caps[SOC_MEMORY_TYPE_NO_PRIOS]; ///< Capabilities for the type of memory in this heap (as a prioritised set). Copied from soc_memory_types so it's in RAM not flash.
diff --git a/components/heap/multi_heap_poisoning.c b/components/heap/multi_heap_poisoning.c
index aa50bc59e..3c8cff241 100644
--- a/components/heap/multi_heap_poisoning.c
+++ b/components/heap/multi_heap_poisoning.c
@@ -175,6 +175,9 @@ static bool verify_fill_pattern(void *data, size_t size, bool print_errors, bool
 
 void *multi_heap_malloc(multi_heap_handle_t heap, size_t size)
 {
+    if(size > SIZE_MAX - POISON_OVERHEAD) {
+        return NULL;
+    }
     multi_heap_internal_lock(heap);
     poison_head_t *head = multi_heap_malloc_impl(heap, size + POISON_OVERHEAD);
     uint8_t *data = NULL;
@@ -217,6 +220,9 @@ void *multi_heap_realloc(multi_heap_handle_t heap, void *p, size_t size)
     poison_head_t *new_head;
     void *result = NULL;
 
+    if(size > SIZE_MAX - POISON_OVERHEAD) {
+        return NULL;
+    }
     if (p == NULL) {
         return multi_heap_malloc(heap, size);
     }
diff --git a/components/heap/test/test_malloc.c b/components/heap/test/test_malloc.c
index 23eef9d8b..3b329e386 100644
--- a/components/heap/test/test_malloc.c
+++ b/components/heap/test/test_malloc.c
@@ -65,7 +65,6 @@ TEST_CASE("Malloc/overwrite, then free all available DRAM", "[heap]")
     TEST_ASSERT(m1==m2);
 }
 
-
 #if CONFIG_SPIRAM_USE_MALLOC
 
 #if (CONFIG_SPIRAM_MALLOC_RESERVE_INTERNAL > 1024)
@@ -87,4 +86,30 @@ TEST_CASE("Check if reserved DMA pool still can allocate even when malloc()'ed m
 }
 #endif
 
-#endif
\ No newline at end of file
+#endif
+
+TEST_CASE("alloc overflows should all fail", "[heap]")
+{
+    /* allocates 8 bytes if size_t overflows */
+    TEST_ASSERT_NULL(calloc(SIZE_MAX / 2 + 4, 2));
+
+    /* will overflow if any poisoning is enabled
+       (should fail for sensible OOM reasons, otherwise) */
+    TEST_ASSERT_NULL(malloc(SIZE_MAX - 1));
+    TEST_ASSERT_NULL(calloc(SIZE_MAX - 1, 1));
+
+    /* will overflow when the size is rounded up to word align it */
+    TEST_ASSERT_NULL(heap_caps_malloc(SIZE_MAX-1, MALLOC_CAP_32BIT));
+
+    TEST_ASSERT_NULL(heap_caps_malloc(SIZE_MAX-1, MALLOC_CAP_EXEC));
+}
+
+TEST_CASE("unreasonable allocs should all fail", "[heap]")
+{
+    TEST_ASSERT_NULL(calloc(16, 1024*1024));
+    TEST_ASSERT_NULL(malloc(16*1024*1024));
+    TEST_ASSERT_NULL(malloc(SIZE_MAX / 2));
+    TEST_ASSERT_NULL(malloc(SIZE_MAX - 256));
+    TEST_ASSERT_NULL(malloc(xPortGetFreeHeapSize() - 1));
+}
+
diff --git a/components/newlib/syscalls.c b/components/newlib/syscalls.c
index f4528f6ea..d3563e247 100644
--- a/components/newlib/syscalls.c
+++ b/components/newlib/syscalls.c
@@ -47,11 +47,17 @@ void* IRAM_ATTR _realloc_r(struct _reent *r, void* ptr, size_t size)
     return heap_caps_realloc_default( ptr, size );
 }
 
-void* IRAM_ATTR _calloc_r(struct _reent *r, size_t count, size_t size)
+void* IRAM_ATTR _calloc_r(struct _reent *r, size_t nmemb, size_t size)
 {
-    void* result = heap_caps_malloc_default(count * size);
-    if (result) {
-        bzero(result, count * size);
+    void *result;
+    size_t size_bytes;
+    if (__builtin_mul_overflow(nmemb, size, &size_bytes)) {
+        return NULL;
+    }
+
+    result = malloc(size_bytes);
+    if (result != NULL) {
+        bzero(result, size_bytes);
     }
     return result;
 }
diff --git a/components/soc/esp32/include/soc/soc.h b/components/soc/esp32/include/soc/soc.h
index 86a94c974..41947847a 100644
--- a/components/soc/esp32/include/soc/soc.h
+++ b/components/soc/esp32/include/soc/soc.h
@@ -63,6 +63,8 @@
 #define SOC_IROM_HIGH   0x40400000
 #define SOC_DROM_LOW    0x3F400000
 #define SOC_DROM_HIGH   0x3F800000
+#define SOC_DRAM_LOW    0x3FAE0000
+#define SOC_DRAM_HIGH   0x40000000
 #define SOC_RTC_IRAM_LOW  0x400C0000
 #define SOC_RTC_IRAM_HIGH 0x400C2000
 #define SOC_RTC_DATA_LOW  0x50000000
@@ -70,6 +72,12 @@
 #define SOC_EXTRAM_DATA_LOW 0x3F800000
 #define SOC_EXTRAM_DATA_HIGH 0x3FC00000
 
+#define SOC_MAX_CONTIGUOUS_RAM_SIZE 0x400000 ///< Largest span of contiguous memory (DRAM or IRAM) in the address space
+
+#define SOC_CACHE_PRO_LOW   0x40070000
+#define SOC_CACHE_PRO_HIGH  0x40078000
+#define SOC_CACHE_APP_LOW   0x40078000
+#define SOC_CACHE_APP_HIGH  0x40080000
 
 #define DR_REG_DPORT_BASE                       0x3ff00000
 #define DR_REG_AES_BASE                         0x3ff01000
diff --git a/components/soc/include/soc/soc_memory_layout.h b/components/soc/include/soc/soc_memory_layout.h
index 6273b1dbc..593d68d9e 100644
--- a/components/soc/include/soc/soc_memory_layout.h
+++ b/components/soc/include/soc/soc_memory_layout.h
@@ -94,3 +94,28 @@ inline static bool IRAM_ATTR esp_ptr_internal(const void *p) {
 inline static bool IRAM_ATTR esp_ptr_external_ram(const void *p) {
     return ((intptr_t)p >= SOC_EXTRAM_DATA_LOW && (intptr_t)p < SOC_EXTRAM_DATA_HIGH);
 }
+
+inline static bool IRAM_ATTR esp_ptr_in_iram(const void *p) {
+#ifndef CONFIG_FREERTOS_UNICORE
+    return ((intptr_t)p >= SOC_IRAM_LOW && (intptr_t)p < SOC_IRAM_HIGH);
+#else
+    return ((intptr_t)p >= SOC_CACHE_APP_LOW && (intptr_t)p < SOC_IRAM_HIGH);
+#endif
+}
+
+inline static bool IRAM_ATTR esp_ptr_in_drom(const void *p) {
+    return ((intptr_t)p >= SOC_DROM_LOW && (intptr_t)p < SOC_DROM_HIGH);
+}
+
+inline static bool IRAM_ATTR esp_ptr_in_dram(const void *p) {
+    return ((intptr_t)p >= SOC_DRAM_LOW && (intptr_t)p < SOC_DRAM_HIGH);
+}
+
+inline static bool IRAM_ATTR esp_ptr_in_diram_dram(const void *p) {
+    return ((intptr_t)p >= SOC_DIRAM_DRAM_LOW && (intptr_t)p < SOC_DIRAM_DRAM_HIGH);
+}
+
+inline static bool IRAM_ATTR esp_ptr_in_diram_iram(const void *p) {
+    return ((intptr_t)p >= SOC_DIRAM_IRAM_LOW && (intptr_t)p < SOC_DIRAM_IRAM_HIGH);
+}
+