commit 3cea86eb2fbb65949673eb4ba8ebb695c87a57ce
Author: Nick Mathewson <ni...@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;
}
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