free_prompts: deal with a reference from an Ldisc.

In a GUI app, when interactive userpass input begins, the Ldisc
acquires a reference to a prompts_t. If something bad happens to the
SSH connection (e.g. unexpected server-side closure), then all the SSH
layers will be destroyed, including freeing that prompts_t. So the
Ldisc will have a stale reference to it, which it might potentially
use.

To fix that, I've arranged a back-pointer so that prompts_t itself can
find the Ldisc's reference to it, and NULL it out on free. So now,
whichever of a prompts_t and an Ldisc is freed first, the link between
them should be cleanly broken.

(I'm not 100% sure this is absolutely necessary, in the sense of
whether a sequence of events can _actually_ happen that causes a stale
pointer dereference. But I don't want to take the chance!)

ldisc.c

@@ -182,6 +182,8 @@ void ldisc_free(Ldisc *ldisc)
         backend_provide_ldisc(ldisc->backend, NULL);
     if (ldisc->buf)
         sfree(ldisc->buf);
+    if (ldisc->prompts && ldisc->prompts->ldisc_ptr_to_us == &ldisc->prompts)
+        ldisc->prompts->ldisc_ptr_to_us = NULL;
     delete_callbacks_for_context(ldisc);
     sfree(ldisc);
 }
@@ -201,6 +203,8 @@ void ldisc_enable_prompt_callback(Ldisc *ldisc, prompts_t *prompts)
      * that it can continue the interactive prompting process.
      */
     ldisc->prompts = prompts;
+    if (prompts)
+        ldisc->prompts->ldisc_ptr_to_us = &ldisc->prompts;
 }
 
 static void ldisc_input_queue_callback(void *ctx)

putty.h

@@ -767,7 +767,8 @@ typedef struct {
     bool echo;
     strbuf *result;
 } prompt_t;
-typedef struct {
+typedef struct prompts_t prompts_t;
+struct prompts_t {
     /*
      * Indicates whether the information entered is to be used locally
      * (for instance a key passphrase prompt), or is destined for the wire.
@@ -806,7 +807,14 @@ typedef struct {
      */
     toplevel_callback_fn_t callback;
     void *callback_ctx;
-} prompts_t;
+
+    /*
+     * When this prompts_t is known to an Ldisc, we might need to
+     * break the connection if things get freed in an emergency. So
+     * this is a pointer to the Ldisc's pointer to us.
+     */
+    prompts_t **ldisc_ptr_to_us;
+};
 prompts_t *new_prompts(void);
 void add_prompt(prompts_t *p, char *promptstr, bool echo);
 void prompt_set_result(prompt_t *pr, const char *newstr);

utils/prompts.c

@@ -17,6 +17,7 @@ prompts_t *new_prompts(void)
     p->name_reqd = p->instr_reqd = false;
     p->callback = NULL;
     p->callback_ctx = NULL;
+    p->ldisc_ptr_to_us = NULL;
     return p;
 }
 
@@ -49,6 +50,12 @@ char *prompt_get_result(prompt_t *pr)
 void free_prompts(prompts_t *p)
 {
     size_t i;
+
+    /* If an Ldisc currently knows about us, tell it to forget us, so
+     * it won't dereference a stale pointer later. */
+    if (p->ldisc_ptr_to_us)
+        *p->ldisc_ptr_to_us = NULL;
+
     for (i=0; i < p->n_prompts; i++) {
         prompt_t *pr = p->prompts[i];
         strbuf_free(pr->result);