There's a reference count leak in aa_fs_seq_hash_show that can be used to overflow the reference counter and trigger a kernel use-after-free

static int aa_fs_seq_hash_show(struct seq_file *seq, void *v)
{
	struct aa_replacedby *r = seq->private;
	struct aa_profile *profile = aa_get_profile_rcu(&r->profile); // <--- takes a reference on profile
	unsigned int i, size = aa_hash_size();

	if (profile->hash) {
		for (i = 0; i < size; i++)
			seq_printf(seq, "%.2x", profile->hash[i]);
		seq_puts(seq, "\n");
	}

	return 0;
} // <-- no reference dropped

See attached for a PoC that triggers a use-after-free on an aa_label object on Ubuntu 15.10 with the latest 4.2.0.35 kernel; the Ubuntu kernel appears to use an older version of AppArmor prior to some refactoring, but the same issue is present.

static int aa_fs_seq_hash_show(struct seq_file *seq, void *v)
{
	struct aa_replacedby *r = seq->private;
	struct aa_label *label = aa_get_label_rcu(&r->label); // <--- takes a reference on label
	struct aa_profile *profile = labels_profile(label);
	unsigned int i, size = aa_hash_size();

	if (profile->hash) {
		for (i = 0; i < size; i++)
			seq_printf(seq, "%.2x", profile->hash[i]);
		seq_puts(seq, "\n");
	}

	return 0;
} // <--- no reference dropped

I noticed in reproducing this issue that it appears that there has been a patch applied to the very latest Ubuntu kernel shipped in 16.04 that fixes this that hasn't been upstreamed or backported.

The fix is just to correctly drop the acquired reference.

index ad4fa49..798d492 100644
--- a/security/apparmor/apparmorfs.c
+++ b/security/apparmor/apparmorfs.c
@@ -331,6 +331,7 @@ static int aa_fs_seq_hash_show(struct seq_file *seq, void *v)
 			seq_printf(seq, "%.2x", profile->hash[i]);
 		seq_puts(seq, "\n");
 	}
+	aa_put_profile(profile);
 
 	return 0;
 }

This bug is subject to a 90 day disclosure deadline. If 90 days elapse
without a broadly available patch, then the bug report will automatically
become visible to the public.
 

apparmor.c 1.4 KB View Download