In ```or/buffer.s.c```: ```c /** Return the allocation size we'd like to use to hold target * bytes. */ static inline size_t preferred_chunk_size(size_t target) { size_t sz = MIN_CHUNK_ALLOC; while (CHUNK_SIZE_WITH_ALLOC(sz) < target) { sz <<= 1; } return sz; } ``` ```c #define MIN_CHUNK_ALLOC 256 #define CHUNK_SIZE_WITH_ALLOC(memlen) ((memlen) - CHUNK_HEADER_LEN) ``` CHUNK_HEADER_LEN is usually around 30 bytes or so. The problem with ```preferred_chunk_size``` is that for a large ```size_t target```, the function will return 0. If you compile this program with ```-m32```: ```c #include #include #define FLEXIBLE_ARRAY_MEMBER /**/ #define DEBUG_CHUNK_ALLOC /** A single chunk on a buffer. */ typedef struct chunk_t { struct chunk_t *next; /**< The next chunk on the buffer. */ size_t datalen; /**< The number of bytes stored in this chunk */ size_t memlen; /**< The number of usable bytes of storage in mem. */ #ifdef DEBUG_CHUNK_ALLOC size_t DBG_alloc; #endif char *data; /**< A pointer to the first byte of data stored in mem. */ uint32_t inserted_time; /**< Timestamp in truncated ms since epoch * when this chunk was inserted. */ char mem[FLEXIBLE_ARRAY_MEMBER]; /**< The actual memory used for storage in * this chunk. */ } chunk_t; #if defined(__GNUC__) && __GNUC__ > 3 #define STRUCT_OFFSET(tp, member) __builtin_offsetof(tp, member) #else #define STRUCT_OFFSET(tp, member) \ ((off_t) (((char*)&((tp*)0)->member)-(char*)0)) #endif #define MIN_CHUNK_ALLOC 256 #define CHUNK_HEADER_LEN STRUCT_OFFSET(chunk_t, mem[0]) #define CHUNK_SIZE_WITH_ALLOC(memlen) ((memlen) - CHUNK_HEADER_LEN) static inline size_t preferred_chunk_size(size_t target) { size_t sz = MIN_CHUNK_ALLOC; while (CHUNK_SIZE_WITH_ALLOC(sz) < target) { sz <<= 1; } return sz; } int main(void) { size_t i = 1024; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; printf("i is %08X, preferred_chunk_size is %08X\n", i, preferred_chunk_size(i)); i <<= 1; return 0; } ``` the output is: ``` i is 00000400, preferred_chunk_size is 00000800 i is 00000800, preferred_chunk_size is 00001000 i is 00001000, preferred_chunk_size is 00002000 i is 00002000, preferred_chunk_size is 00004000 i is 00004000, preferred_chunk_size is 00008000 i is 00008000, preferred_chunk_size is 00010000 i is 00010000, preferred_chunk_size is 00020000 i is 00020000, preferred_chunk_size is 00040000 i is 00040000, preferred_chunk_size is 00080000 i is 00080000, preferred_chunk_size is 00100000 i is 00100000, preferred_chunk_size is 00200000 i is 00200000, preferred_chunk_size is 00400000 i is 00400000, preferred_chunk_size is 00800000 i is 00800000, preferred_chunk_size is 01000000 i is 01000000, preferred_chunk_size is 02000000 i is 02000000, preferred_chunk_size is 04000000 i is 04000000, preferred_chunk_size is 08000000 i is 08000000, preferred_chunk_size is 10000000 i is 10000000, preferred_chunk_size is 20000000 i is 20000000, preferred_chunk_size is 40000000 i is 40000000, preferred_chunk_size is 80000000 i is 80000000, preferred_chunk_size is 00000000 ``` The danger is that the return value of ```preferred_chunk_size``` is always used as a parameter to ```tor_malloc``` or ```tor_realloc```. It is called at these places: In ```buf_pullup```: ```c 210 newsize = CHUNK_SIZE_WITH_ALLOC(preferred_chunk_size(capacity)); 211 newhead = chunk_grow(buf->head, newsize); ``` In ```buf_new_with_capacity```: ```c 283 /** Create and return a new buf with default chunk capacity size. 284 */ 285 buf_t * 286 buf_new_with_capacity(size_t size) 287 { 288 buf_t *b = buf_new(); 289 b->default_chunk_size = preferred_chunk_size(size); 290 return b; 291 } ``` In ```buf_add_chunk_with_capacity```: ```c 401 /** Append a new chunk with enough capacity to hold capacity bytes to 402 * the tail of buf. If capped, don't allocate a chunk bigger 403 * than MAX_CHUNK_ALLOC. */ 404 static chunk_t * 405 buf_add_chunk_with_capacity(buf_t *buf, size_t capacity, int capped) 406 { 407 chunk_t *chunk; 408 409 if (CHUNK_ALLOC_SIZE(capacity) < buf->default_chunk_size) { 410 chunk = chunk_new_with_alloc_size(buf->default_chunk_size); 411 } else if (capped && CHUNK_ALLOC_SIZE(capacity) > MAX_CHUNK_ALLOC) { 412 chunk = chunk_new_with_alloc_size(MAX_CHUNK_ALLOC); 413 } else { 414 chunk = chunk_new_with_alloc_size(preferred_chunk_size(capacity)); 415 } ``` ```buf_new_with_capacity``` is currently called nowhere except for tests. ```buf_add_chunk_with_capacity``` is called at various places but currently not with the ```capped``` parameter set to 0. However, ```buf_pullup``` is called at various places and the call to ```preferred_chunk_size``` is reachable. Whether it is reachable with a parameter large enough that it will return 0 I'm not sure about. ```c int tor_main(int argc, char *argv[]) { buf_t* buf; char* string; size_t string_len; size_t i; buf = buf_new(); string_len = 0x00001000; string = tor_malloc(string_len); for (i = 0; i < 507904; i++) { write_to_buf(string, string_len, buf); } write_to_buf(string, 0x3FFFFFA, buf); free(string); buf_pullup(buf, 0x90000000); } ``` What will happen is that ```buf_pullup``` will call ```chunk_grow``` ```c 140 static inline chunk_t * 141 chunk_grow(chunk_t *chunk, size_t sz) 142 { 143 off_t offset; 144 size_t memlen_orig = chunk->memlen; 145 tor_assert(sz > chunk->memlen); 146 offset = chunk->data - chunk->mem; 147 chunk = tor_realloc(chunk, CHUNK_ALLOC_SIZE(sz)); 148 chunk->memlen = sz; 149 chunk->data = chunk->mem + offset; ``` ```tor_realloc``` will in effect be called with a size parameter of 0. Whether and how much legitimate heap memory ```realloc``` will allocate might be implementation-dependent. The point is that the following lines might overwrite heap memory: ```c 148 chunk->memlen = sz; 149 chunk->data = chunk->mem + offset; ``` since ```chunk``` is a memory area that has just been allocated to 0 (or 1, after correction) bytes. The whole scenario is not very likely considering Tor's frugal memory consumption but it is nonetheless a programming fault in the buffers "API" that could lead to stability issues. Especially if you ever expand the use of ```buf_pullup```, ```buf_new_with_capacity```, and/or uncapped ```buf_add_chunk_with_capacity```, it'll be wise to hard-limit the amounts of right-shifts in ```preferred_chunk_size``` (a single unintended negative integer -> size_t can be conducive in establishing an exploitation path).

Tor: potential memory corruption in or/buffers.c (particularly on 32 bit)