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

[Hao Ge <hao.ge@linux.dev>]

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.

Fix by always reinitializing the iterator in allocinfo_start().

Fixes: 9f44df50fee4 ("alloc_tag: keep codetag iterator active between read()")
Signed-off-by: Hao Ge <hao.ge@linux.dev>
---
 lib/alloc_tag.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index ed1bdcf1f8ab..2b2d1580c714 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -51,16 +51,19 @@ struct allocinfo_private {
 static void *allocinfo_start(struct seq_file *m, loff_t *pos)
 {
 	struct allocinfo_private *priv;
+	struct codetag *ct;
 	loff_t node = *pos;
 
 	priv = (struct allocinfo_private *)m->private;
 	codetag_lock_module_list(alloc_tag_cttype, true);
-	if (node == 0) {
+	if (node == 0)
 		priv->print_header = true;
-		priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
-		codetag_next_ct(&priv->iter);
-	}
-	return priv->iter.ct ? priv : NULL;
+
+	priv->iter = codetag_get_ct_iter(alloc_tag_cttype);
+	while ((ct = codetag_next_ct(&priv->iter)) != NULL && node)
+		node--;
+
+	return ct ? priv : NULL;
 }
 
 static void *allocinfo_next(struct seq_file *m, void *arg, loff_t *pos)
-- 
2.25.1