commit 3cea86eb2fbb65949673eb4ba8ebb695c87a57ce Author: Nick Mathewson <nickm@torproject.org> Date: Fri Oct 14 09:38:12 2016 -0400 Add a one-word sentinel value of 0x0 at the end of each buf_t chunk This helps protect against bugs where any part of a buf_t's memory is passed to a function that expects a NUL-terminated input. It also closes TROVE-2016-10-001 (aka bug 20384). --- changes/buf-sentinel | 11 +++++++++++ src/or/buffers.c | 40 ++++++++++++++++++++++++++++++++-------- 2 files changed, 43 insertions(+), 8 deletions(-) diff --git a/changes/buf-sentinel b/changes/buf-sentinel new file mode 100644 index 0000000..7c5b829 --- /dev/null +++ b/changes/buf-sentinel @@ -0,0 +1,11 @@ + o Major features (security fixes): + + - Prevent a class of security bugs caused by treating the contents + of a buffer chunk as if they were a NUL-terminated string. At + least one such bug seems to be present in all currently used + versions of Tor, and would allow an attacker to remotely crash + most Tor instances, especially those compiled with extra compiler + hardening. With this defense in place, such bugs can't crash Tor, + though we should still fix them as they occur. Closes ticket 20384 + (TROVE-2016-10-001). + diff --git a/src/or/buffers.c b/src/or/buffers.c index be99744..74aebcc 100644 --- a/src/or/buffers.c +++ b/src/or/buffers.c @@ -69,12 +69,33 @@ static int parse_socks_client(const uint8_t *data, size_t datalen, #define CHUNK_HEADER_LEN STRUCT_OFFSET(chunk_t, mem[0]) +/* We leave this many NUL bytes at the end of the buffer. */ +#define SENTINEL_LEN 4 + +/* Header size plus NUL bytes at the end */ +#define CHUNK_OVERHEAD (CHUNK_HEADER_LEN + SENTINEL_LEN) + /** Return the number of bytes needed to allocate a chunk to hold * <b>memlen</b> bytes. */ -#define CHUNK_ALLOC_SIZE(memlen) (CHUNK_HEADER_LEN + (memlen)) +#define CHUNK_ALLOC_SIZE(memlen) (CHUNK_OVERHEAD + (memlen)) /** Return the number of usable bytes in a chunk allocated with * malloc(<b>memlen</b>). */ -#define CHUNK_SIZE_WITH_ALLOC(memlen) ((memlen) - CHUNK_HEADER_LEN) +#define CHUNK_SIZE_WITH_ALLOC(memlen) ((memlen) - CHUNK_OVERHEAD) + +#define DEBUG_SENTINEL + +#ifdef DEBUG_SENTINEL +#define DBG_S(s) s +#else +#define DBG_S(s) (void)0 +#endif + +#define CHUNK_SET_SENTINEL(chunk, alloclen) do { \ + uint8_t *a = (uint8_t*) &(chunk)->mem[(chunk)->memlen]; \ + DBG_S(uint8_t *b = &((uint8_t*)(chunk))[(alloclen)-SENTINEL_LEN]); \ + DBG_S(tor_assert(a == b)); \ + memset(a,0,SENTINEL_LEN); \ + } while (0) /** Return the next character in <b>chunk</b> onto which data can be appended. * If the chunk is full, this might be off the end of chunk->mem. */ @@ -131,6 +152,7 @@ chunk_new_with_alloc_size(size_t alloc) ch->memlen = CHUNK_SIZE_WITH_ALLOC(alloc); total_bytes_allocated_in_chunks += alloc; ch->data = &ch->mem[0]; + CHUNK_SET_SENTINEL(ch, alloc); return ch; } @@ -140,18 +162,20 @@ static INLINE chunk_t * chunk_grow(chunk_t *chunk, size_t sz) { off_t offset; - size_t memlen_orig = chunk->memlen; + const size_t memlen_orig = chunk->memlen; + const size_t orig_alloc = CHUNK_ALLOC_SIZE(memlen_orig); + const size_t new_alloc = CHUNK_ALLOC_SIZE(sz); tor_assert(sz > chunk->memlen); offset = chunk->data - chunk->mem; - chunk = tor_realloc(chunk, CHUNK_ALLOC_SIZE(sz)); + chunk = tor_realloc(chunk, new_alloc); chunk->memlen = sz; chunk->data = chunk->mem + offset; #ifdef DEBUG_CHUNK_ALLOC - tor_assert(chunk->DBG_alloc == CHUNK_ALLOC_SIZE(memlen_orig)); - chunk->DBG_alloc = CHUNK_ALLOC_SIZE(sz); + tor_assert(chunk->DBG_alloc == orig_alloc); + chunk->DBG_alloc = new_alloc; #endif - total_bytes_allocated_in_chunks += - CHUNK_ALLOC_SIZE(sz) - CHUNK_ALLOC_SIZE(memlen_orig); + total_bytes_allocated_in_chunks += new_alloc - orig_alloc; + CHUNK_SET_SENTINEL(chunk, new_alloc); return chunk; }