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free_prompts: deal with a reference from an Ldisc.

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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!)
This commit is contained in:
Simon Tatham
2021-09-16 09:41:03 +01:00
parent 7a02234353
commit 65270b56f0
3 changed files with 21 additions and 2 deletions
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12
putty.h
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@ -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);