hs: Fix bad use of sizeof() when encoding ESTABLISH_INTRO legacy cell

When encoding a legacy ESTABLISH_INTRO cell, we were using the sizeof() on a
pointer instead of using the real size of the destination buffer leading to an
overflow passing an enormous value to the signing digest function.
Fortunately, that value was only used to make sure the destination buffer
length was big enough for the key size and in this case it always was because
of the overflow.

Fixes #21553

Signed-off-by: David Goulet <dgoulet@torproject.org>

4 files changed, 16 insertions, 5 deletions

diff --git a/changes/bug21553 b/changes/bug21553
new file mode 100644
index 0000000000..6ffa3e29a2
--- /dev/null
+++ b/changes/bug21553
@@ -0,0 +1,7 @@
+  o Minor bugfixes (hidden service):
+    - When encoding a legacy ESTABLISH_INTRO cell, we were using the sizeof()
+      on a pointer instead of real size of the destination buffer leading to
+      an overflow passing an enormous value to the signing digest function.
+      Fortunately, that value was only used to make sure the destination
+      buffer length was big enough for the key size and in this case it was.
+      Fixes bug 21553; bugfix on tor-0.3.0.1-alpha.
diff --git a/src/or/rendservice.c b/src/or/rendservice.c
index 1d6fc0f96d..522f33e5bb 100644
--- a/src/or/rendservice.c
+++ b/src/or/rendservice.c
@@ -3174,8 +3174,9 @@ count_intro_point_circuits(const rend_service_t *service)
    of bytes written. On fail, return -1.
  */
 STATIC ssize_t
-encode_establish_intro_cell_legacy(char *cell_body_out, crypto_pk_t *intro_key,
-                                   char *rend_circ_nonce)
+encode_establish_intro_cell_legacy(char *cell_body_out,
+                                   size_t cell_body_out_len,
+                                   crypto_pk_t *intro_key, char *rend_circ_nonce)
 {
   int retval = -1;
   int r;
@@ -3202,7 +3203,7 @@ encode_establish_intro_cell_legacy(char *cell_body_out, crypto_pk_t *intro_key,
   len += 20;
   note_crypto_pk_op(REND_SERVER);
   r = crypto_pk_private_sign_digest(intro_key, cell_body_out+len,
-                                    sizeof(cell_body_out)-len,
+                                    cell_body_out_len - len,
                                     cell_body_out, len);
   if (r<0) {
     log_warn(LD_BUG, "Internal error: couldn't sign introduction request.");
@@ -3313,8 +3314,9 @@ rend_service_intro_has_opened(origin_circuit_t *circuit)
   /* Send the ESTABLISH_INTRO cell */
   {
     ssize_t len;
-    len = encode_establish_intro_cell_legacy(buf, circuit->intro_key,
-                                        circuit->cpath->prev->rend_circ_nonce);
+    len = encode_establish_intro_cell_legacy(buf, sizeof(buf),
+                                      circuit->intro_key,
+                                      circuit->cpath->prev->rend_circ_nonce);
     if (len < 0) {
       reason = END_CIRC_REASON_INTERNAL;
       goto err;
diff --git a/src/or/rendservice.h b/src/or/rendservice.h
index 3bfac0bece..85daaae4e2 100644
--- a/src/or/rendservice.h
+++ b/src/or/rendservice.h
@@ -130,6 +130,7 @@ STATIC int rend_service_poison_new_single_onion_dir(
                                                   const rend_service_t *s,
                                                   const or_options_t* options);
 STATIC ssize_t encode_establish_intro_cell_legacy(char *cell_body_out,
+                                                  size_t cell_body_out_len,
                                                   crypto_pk_t *intro_key,
                                                   char *rend_circ_nonce);
 STATIC void prune_services_on_reload(smartlist_t *old_service_list,
diff --git a/src/test/test_hs_intropoint.c b/src/test/test_hs_intropoint.c
index ea12aeb2da..b207cd4ce3 100644
--- a/src/test/test_hs_intropoint.c
+++ b/src/test/test_hs_intropoint.c
@@ -489,6 +489,7 @@ helper_establish_intro_v2(or_circuit_t *intro_circ)
 
   /* Use old circuit_key_material why not */
   cell_len = encode_establish_intro_cell_legacy((char*)cell_body,
+                                                sizeof(cell_body),
                                                 key1,
                                                 (char *) circuit_key_material);
   tt_int_op(cell_len, >, 0);