alloc_tag: fix use-after-free in /proc/allocinfo after module unload

allocinfo_start() only reinitializes the codetag iterator at position 0. 
For subsequent reads (position > 0), it reuses cached iterator state from
the previous batch.  allocinfo_stop() drops mod_lock between read batches,
which allows module unload to complete and free the module memory that the
cached iterator still references:

  CPU0 (read)                        CPU1 (rmmod)
  ----                               ----
  allocinfo_start(pos=0)
    down_read(mod_lock)
    allocinfo_show()
    ...
  allocinfo_stop()
    up_read(mod_lock)
                                     codetag_unload_module()
                                       kfree(cmod)
                                       release_module_tags()
                                     ...
                                     free_mod_mem()
  allocinfo_start(pos=N)
    down_read(mod_lock)
    // reuses cached iter, skips re-init
  allocinfo_show()
    ct->filename   <-- UAF

After free_mod_mem() frees the module's .rodata, allocinfo_show()
dereferences ct->filename, ct->function which point there.

Save the iterator state in allocinfo_next() and resume from it in
allocinfo_start() with codetag_next_ct(), which detects module removal via
idr_find() returning NULL and skips to the next module.

Link: https://lore.kernel.org/20260604065938.105991-1-hao.ge@linux.dev
Fixes: 9f44df50fe ("alloc_tag: keep codetag iterator active between read()")
Signed-off-by: Hao Ge <hao.ge@linux.dev>
Suggested-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Suren Baghdasaryan <surenb@google.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/alloc_tag.c

@@ -45,6 +45,7 @@ int alloc_tag_ref_offs;
 
 struct allocinfo_private {
 	struct codetag_iterator iter;
+	struct codetag_iterator reported_iter;
 	bool print_header;
 };
 
@@ -58,16 +59,20 @@ static void *allocinfo_start(struct seq_file *m, loff_t *pos)
 	if (node == 0) {
 		priv->print_header = true;
 		priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
-		codetag_next_ct(&priv->iter);
+	} else {
+		priv->iter = priv->reported_iter;
 	}
+	codetag_next_ct(&priv->iter);
 	return priv->iter.ct ? priv : NULL;
 }
 
 static void *allocinfo_next(struct seq_file *m, void *arg, loff_t *pos)
 {
 	struct allocinfo_private *priv = (struct allocinfo_private *)arg;
-	struct codetag *ct = codetag_next_ct(&priv->iter);
+	struct codetag *ct;
 
+	priv->reported_iter = priv->iter;
+	ct = codetag_next_ct(&priv->iter);
 	(*pos)++;
 	if (!ct)
 		return NULL;