bpf: Fix KASAN use-after-free Read in compute_effective_progs

Syzbot found a Use After Free bug in compute_effective_progs().
The reproducer creates a number of BPF links, and causes a fault
injected alloc to fail, while calling bpf_link_detach on them.
Link detach triggers the link to be freed by bpf_link_free(),
which calls __cgroup_bpf_detach() and update_effective_progs().
If the memory allocation in this function fails, the function restores
the pointer to the bpf_cgroup_link on the cgroup list, but the memory
gets freed just after it returns. After this, every subsequent call to
update_effective_progs() causes this already deallocated pointer to be
dereferenced in prog_list_length(), and triggers KASAN UAF error.

To fix this issue don't preserve the pointer to the prog or link in the
list, but remove it and replace it with a dummy prog without shrinking
the table. The subsequent call to __cgroup_bpf_detach() or
__cgroup_bpf_detach() will correct it.

Cc: "Alexei Starovoitov" <ast@kernel.org>
Cc: "Daniel Borkmann" <daniel@iogearbox.net>
Cc: "Andrii Nakryiko" <andrii@kernel.org>
Cc: "Martin KaFai Lau" <kafai@fb.com>
Cc: "Song Liu" <songliubraving@fb.com>
Cc: "Yonghong Song" <yhs@fb.com>
Cc: "John Fastabend" <john.fastabend@gmail.com>
Cc: "KP Singh" <kpsingh@kernel.org>
Cc: <netdev@vger.kernel.org>
Cc: <bpf@vger.kernel.org>
Cc: <stable@vger.kernel.org>
Cc: <linux-kernel@vger.kernel.org>

Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
Fixes: af6eea5 ("bpf: Implement bpf_link-based cgroup BPF program attachment")
Reported-by: <syzbot+f264bffdfbd5614f3bb2@syzkaller.appspotmail.com>
Signed-off-by: Tadeusz Struk <tadeusz.struk@linaro.org>

kernel/bpf/cgroup.c

@@ -720,6 +720,60 @@ static struct bpf_prog_list *find_detach_entry(struct list_head *progs,
 	return ERR_PTR(-ENOENT);
 }
 
+/**
+ * purge_effective_progs() - After compute_effective_progs fails to alloc new
+ *                           cgrp->bpf.inactive table we can recover by
+ *                           recomputing the array in place.
+ *
+ * @cgrp: The cgroup which descendants to travers
+ * @prog: A program to detach or NULL
+ * @link: A link to detach or NULL
+ * @atype: Type of detach operation
+ */
+static void purge_effective_progs(struct cgroup *cgrp, struct bpf_prog *prog,
+				  struct bpf_cgroup_link *link,
+				  enum cgroup_bpf_attach_type atype)
+{
+	struct cgroup_subsys_state *css;
+	struct bpf_prog_array *progs;
+	struct bpf_prog_list *pl;
+	struct list_head *head;
+	struct cgroup *cg;
+	int pos;
+
+	/* recompute effective prog array in place */
+	css_for_each_descendant_pre(css, &cgrp->self) {
+		struct cgroup *desc = container_of(css, struct cgroup, self);
+
+		if (percpu_ref_is_zero(&desc->bpf.refcnt))
+			continue;
+
+		/* find position of link or prog in effective progs array */
+		for (pos = 0, cg = desc; cg; cg = cgroup_parent(cg)) {
+			if (pos && !(cg->bpf.flags[atype] & BPF_F_ALLOW_MULTI))
+				continue;
+
+			head = &cg->bpf.progs[atype];
+			list_for_each_entry(pl, head, node) {
+				if (!prog_list_prog(pl))
+					continue;
+				if (pl->prog == prog && pl->link == link)
+					goto found;
+				pos++;
+			}
+		}
+found:
+		BUG_ON(!cg);
+		progs = rcu_dereference_protected(
+				desc->bpf.effective[atype],
+				lockdep_is_held(&cgroup_mutex));
+
+		/* Remove the program from the array */
+		WARN_ONCE(bpf_prog_array_delete_safe_at(progs, pos),
+			  "Failed to purge a prog from array at index %d", pos);
+	}
+}
+
 /**
  * __cgroup_bpf_detach() - Detach the program or link from a cgroup, and
  *                         propagate the change to descendants
@@ -762,8 +816,12 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 	pl->link = NULL;
 
 	err = update_effective_progs(cgrp, atype);
-	if (err)
-		goto cleanup;
+	if (err) {
+		/* If update affective array failed replace the prog with a dummy prog*/
+		pl->prog = old_prog;
+		pl->link = link;
+		purge_effective_progs(cgrp, old_prog, link, atype);
+	}
 
 	/* now can actually delete it from this cgroup list */
 	list_del(&pl->node);
@@ -775,12 +833,6 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 		bpf_prog_put(old_prog);
 	static_branch_dec(&cgroup_bpf_enabled_key[atype]);
 	return 0;
-
-cleanup:
-	/* restore back prog or link */
-	pl->prog = old_prog;
-	pl->link = link;
-	return err;
 }
 
 static int cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,