[PATCH] ftrace: fix race in __modify_ftrace_direct() between tmp_ops registration and direct_functions update

[Andrii Kuchmenko <capyenglishlite@gmail.com>]

In __modify_ftrace_direct(), register_ftrace_function_nolock() makes
tmp_ops visible in ftrace_ops_list before entry->direct is updated
under ftrace_lock. During this window any CPU entering the traced
function calls call_direct_funcs(), reads the old address from
direct_functions via RCU, and jumps to it via
arch_ftrace_set_direct_caller(). If the caller freed or invalidated
the old trampoline before calling modify_ftrace_direct(), this is a
use-after-free in executable code context.

The race window:

  CPU 0 (__modify_ftrace_direct)       CPU 1 (executing traced func)
  ──────────────────────────────       ──────────────────────────────
  register_ftrace_function_nolock()
    -> tmp_ops visible in ops_list
                                        call_direct_funcs()
                                          ftrace_find_rec_direct() -> old_addr
                                          arch_ftrace_set_direct_caller(old_addr)
                                          jump to old_addr  <- UAF if freed
  mutex_lock(&ftrace_lock)
  entry->direct = addr   <- too late
  mutex_unlock(&ftrace_lock)

Fix: update entry->direct under ftrace_lock BEFORE registering tmp_ops.
Any CPU that observes tmp_ops in ftrace_ops_list after this point will
already see the new address when it calls ftrace_find_rec_direct().
Add smp_wmb() between the store and the registration to ensure the
write is visible on weakly-ordered architectures before tmp_ops
becomes observable via ftrace_ops_list.

On error from register_ftrace_function_nolock(), restore entry->direct
to old_addr since tmp_ops never became visible to other CPUs.

This affects all callers of __modify_ftrace_direct(), including:
  - modify_ftrace_direct() used by kernel modules and live patching
  - modify_ftrace_direct_nolock() used by BPF trampolines
    (kernel/bpf/trampoline.c) reachable with CAP_BPF + CAP_PERFMON

Fixes: 0567d6809440 ("ftrace: Add modify_ftrace_direct()")
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Andrii Kuchmenko <capyenglishlite@gmail.com>
---
 kernel/trace/ftrace.c | 35 +++++++++++++++++++++++++----------
 1 file changed, 25 insertions(+), 10 deletions(-)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index a1b2c3d4e5f6..b7c8d9e0f1a2 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -5950,6 +5950,7 @@ static int __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 	struct ftrace_func_entry *entry;
 	struct ftrace_ops tmp_ops;
+	unsigned long old_addr;
 	int err;
 
 	lockdep_assert_held(&direct_mutex);
@@ -5960,22 +5961,36 @@ static int __modify_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
 	if (!entry)
 		return -ENODEV;
 
-	/*
-	 * tmp_ops is registered into ftrace_ops_list here, making it
-	 * visible to all CPUs executing the traced function. However,
-	 * entry->direct is not updated until after this call returns,
-	 * leaving a window where CPUs read the stale (possibly freed)
-	 * direct call address via ftrace_find_rec_direct().
-	 */
-	err = register_ftrace_function_nolock(&tmp_ops);
-	if (err)
-		return err;
-
+	/* Save old address in case we need to roll back on error. */
+	old_addr = entry->direct;
+
+	/*
+	 * Update entry->direct BEFORE registering tmp_ops into
+	 * ftrace_ops_list. This closes the race window where a CPU
+	 * executing the traced function could read the old (potentially
+	 * freed) direct call address between tmp_ops becoming visible
+	 * and entry->direct being updated.
+	 *
+	 * Any CPU that observes tmp_ops in ftrace_ops_list after the
+	 * smp_wmb() below is guaranteed to see the new address when
+	 * it calls ftrace_find_rec_direct().
+	 */
 	mutex_lock(&ftrace_lock);
 	entry->direct = addr;
 	mutex_unlock(&ftrace_lock);
 
+	/*
+	 * Ensure entry->direct store is ordered before tmp_ops
+	 * becomes visible via ftrace_ops_list on weakly-ordered archs.
+	 */
+	smp_wmb();
+
+	err = register_ftrace_function_nolock(&tmp_ops);
+	if (err) {
+		/* tmp_ops never became visible; safe to restore old_addr. */
+		mutex_lock(&ftrace_lock);
+		entry->direct = old_addr;
+		mutex_unlock(&ftrace_lock);
+		return err;
+	}
+
 	/*
 	 * Now that tmp_ops is registered and entry->direct is updated,
 	 * unregister the original ops and clean up.
-- 
2.39.0