CVE-2016-8860: Improper Restriction of Operations within the Bounds of a Memory Buffer (CWE-119)
value: Tor before 0.2.8.9 and 0.2.9.x before 0.2.9.4-alpha had internal functions that were entitled to expect that buf_t data had NUL termination, but the implementation of or/buffers.c did not ensure that NUL termination was present, which allows remote attackers to cause a denial of service (client, hidden service, relay, or authority crash) via crafted data.
CWE description: The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.
CVSS v3 base score: 7.5
Published: 2017-01-04T20:59Z
Repository: https://github.com/torproject/tor
Language: Other
Files changed: changes/buf-sentinel, src/or/buffers.c
Commit date: 2016-10-17 14:49:54 -0400
Commit message: 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).

Diff: diff --git a/changes/buf-sentinel b/changes/buf-sentinel
new file mode 100644
index 000000000..7c5b829c1
--- /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 be9974418..74aebccb7 100644
--- a/src/or/buffers.c
+++ b/src/or/buffers.c
@@ -67,16 +67,37 @@ static int parse_socks_client(const uint8_t *data, size_t datalen,
 
 /* Chunk manipulation functions */
 
 #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. */
 static INLINE char *
 CHUNK_WRITE_PTR(chunk_t *chunk)
@@ -129,31 +150,34 @@ chunk_new_with_alloc_size(size_t alloc)
   ch->DBG_alloc = alloc;
 #endif
   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;
 }
 
 /** Expand <b>chunk</b> until it can hold <b>sz</b> bytes, and return a
  * new pointer to <b>chunk</b>.  Old pointers are no longer valid. */
 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;
 }
 
 /** If a read onto the end of a chunk would be smaller than this number, then
  * just start a new chunk. */

Vulnerable code: #define CHUNK_ALLOC_SIZE(memlen) (CHUNK_HEADER_LEN + (memlen))
#define CHUNK_SIZE_WITH_ALLOC(memlen) ((memlen) - CHUNK_HEADER_LEN)
  size_t memlen_orig = chunk->memlen;
  chunk = tor_realloc(chunk, CHUNK_ALLOC_SIZE(sz));
  tor_assert(chunk->DBG_alloc == CHUNK_ALLOC_SIZE(memlen_orig));
  chunk->DBG_alloc = CHUNK_ALLOC_SIZE(sz);
  total_bytes_allocated_in_chunks +=
    CHUNK_ALLOC_SIZE(sz) - CHUNK_ALLOC_SIZE(memlen_orig);
Fixed code:   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).

/* 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)

#define CHUNK_ALLOC_SIZE(memlen) (CHUNK_OVERHEAD + (memlen))
#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)
  CHUNK_SET_SENTINEL(ch, alloc);
  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);
  chunk = tor_realloc(chunk, new_alloc);
  tor_assert(chunk->DBG_alloc == orig_alloc);
  chunk->DBG_alloc = new_alloc;
  total_bytes_allocated_in_chunks += new_alloc - orig_alloc;
  CHUNK_SET_SENTINEL(chunk, new_alloc);