ftrace: bpf: Fix IPMODIFY + DIRECT in modify_ftrace_direct()

ftrace_hash_ipmodify_enable() checks IPMODIFY and DIRECT ftrace_ops on
the same kernel function. When needed, ftrace_hash_ipmodify_enable()
calls ops->ops_func() to prepare the direct ftrace (BPF trampoline) to
share the same function as the IPMODIFY ftrace (livepatch).

ftrace_hash_ipmodify_enable() is called in register_ftrace_direct() path,
but not called in modify_ftrace_direct() path. As a result, the following
operations will break livepatch:

1. Load livepatch to a kernel function;
2. Attach fentry program to the kernel function;
3. Attach fexit program to the kernel function.

After 3, the kernel function being used will not be the livepatched
version, but the original version.

Fix this by adding ftrace_hash_ipmodify_enable() to modify_ftrace_direct()
and adjust some logic around the call.

Signed-off-by: Song Liu <[email protected]>

Author: liu-song-6

kernel/bpf/trampoline.c

@@ -221,6 +221,13 @@ static int register_fentry(struct bpf_trampoline *tr, void *new_addr)
 
 	if (tr->func.ftrace_managed) {
 		ftrace_set_filter_ip(tr->fops, (unsigned long)ip, 0, 1);
+		/*
+		 * Clearing fops->trampoline_mutex and fops->NULL is
+		 * needed by the "goto again" case in
+		 * bpf_trampoline_update().
+		 */
+		tr->fops->trampoline = 0;
+		tr->fops->func = NULL;
 		ret = register_ftrace_direct(tr->fops, (long)new_addr);
 	} else {
 		ret = bpf_arch_text_poke(ip, BPF_MOD_CALL, NULL, new_addr);
@@ -479,11 +486,6 @@ static int bpf_trampoline_update(struct bpf_trampoline *tr, bool lock_direct_mut
 		 * BPF_TRAMP_F_SHARE_IPMODIFY is set, we can generate the
 		 * trampoline again, and retry register.
 		 */
-		/* reset fops->func and fops->trampoline for re-register */
-		tr->fops->func = NULL;
-		tr->fops->trampoline = 0;
-
-		/* free im memory and reallocate later */
 		bpf_tramp_image_free(im);
 		goto again;
 	}

kernel/trace/ftrace.c

@@ -2020,8 +2020,6 @@ static int __ftrace_hash_update_ipmodify(struct ftrace_ops *ops,
 				if (is_ipmodify)
 					goto rollback;
 
-				FTRACE_WARN_ON(rec->flags & FTRACE_FL_DIRECT);
-
 				/*
 				 * Another ops with IPMODIFY is already
 				 * attached. We are now attaching a direct
@@ -6128,6 +6126,15 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 	if (err)
 		return err;
 
+	/*
+	 * Call ftrace_hash_ipmodify_enable() here, so that we can call
+	 * ops->ops_func for the ops. This is needed because the above
+	 * register_ftrace_function_nolock() worked on tmp_ops.
+	 */
+	err = ftrace_hash_ipmodify_enable(ops);
+	if (err)
+		goto out;
+
 	/*
 	 * Now the ftrace_ops_list_func() is called to do the direct callers.
 	 * We can safely change the direct functions attached to each entry.
@@ -6149,6 +6156,7 @@ __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 
 	mutex_unlock(&ftrace_lock);
 
+out:
 	/* Removing the tmp_ops will add the updated direct callers to the functions */
 	unregister_ftrace_function(&tmp_ops);