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 don't preserve the pointer to the link on the cgroup list
in __cgroup_bpf_detach(), but proceed with the cleanup and retry calling
update_effective_progs() again afterwards.

Cc: "Alexei Starovoitov" <[email protected]>
Cc: "Daniel Borkmann" <[email protected]>
Cc: "Andrii Nakryiko" <[email protected]>
Cc: "Martin KaFai Lau" <[email protected]>
Cc: "Song Liu" <[email protected]>
Cc: "Yonghong Song" <[email protected]>
Cc: "John Fastabend" <[email protected]>
Cc: "KP Singh" <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>
Cc: <[email protected]>

Link: https://syzkaller.appspot.com/bug?id=8ebf179a95c2a2670f7cf1ba62429ec044369db4
Fixes: af6eea5 ("bpf: Implement bpf_link-based cgroup BPF program attachment")
Reported-by: <[email protected]>
Signed-off-by: Tadeusz Struk <[email protected]>

Author: tstruk

kernel/bpf/cgroup.c

@@ -723,10 +723,11 @@ 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;
-
-	/* now can actually delete it from this cgroup list */
+	/*
+	 * Proceed regardless of error. The link and/or prog will be freed
+	 * just after this function returns so just delete it from this
+	 * cgroup list and retry calling update_effective_progs again later.
+	 */
 	list_del(&pl->node);
 	kfree(pl);
 	if (list_empty(progs))
@@ -735,12 +736,11 @@ static int __cgroup_bpf_detach(struct cgroup *cgrp, struct bpf_prog *prog,
 	if (old_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;
+	/* In case of error call update_effective_progs again */
+	if (err)
+		err = update_effective_progs(cgrp, atype);
+
 	return err;
 }
 
@@ -881,6 +881,7 @@ static void bpf_cgroup_link_release(struct bpf_link *link)
 	struct bpf_cgroup_link *cg_link =
 		container_of(link, struct bpf_cgroup_link, link);
 	struct cgroup *cg;
+	int err;
 
 	/* link might have been auto-detached by dying cgroup already,
 	 * in that case our work is done here
@@ -896,8 +897,10 @@ static void bpf_cgroup_link_release(struct bpf_link *link)
 		return;
 	}
 
-	WARN_ON(__cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
-				    cg_link->type));
+	err = __cgroup_bpf_detach(cg_link->cgroup, NULL, cg_link,
+				  cg_link->type);
+	if (err)
+		pr_warn("cgroup_bpf_detach() failed, err %d\n", err);
 
 	cg = cg_link->cgroup;
 	cg_link->cgroup = NULL;