[PATCH v3 2/4] seccomp: release filter after task is fully dead

[Christian Brauner]

The seccomp filter used to be released in free_task() which is called
asynchronously via call_rcu() and assorted mechanisms. Since we need
to inform tasks waiting on the seccomp notifier when a filter goes empty
we will notify them as soon as a task has been marked fully dead in
release_task(). To not split seccomp cleanup into two parts, move
filter release out of free_task() and into release_task() after we've
unhashed struct task from struct pid, exited signals, and unlinked it
from the threadgroups' thread list. We'll put the empty filter
notification infrastructure into it in a follow up patch.

This also renames put_seccomp_filter() to seccomp_filter_release() which
is a more descriptive name of what we're doing here especially once
we've added the empty filter notification mechanism in there.

We're also NULL-ing the task's filter tree entrypoint which seems
cleaner than leaving a dangling pointer in there. Note that this shouldn't
need any memory barriers since we're calling this when the task is in
release_task() which means it's EXIT_DEAD. So it can't modify it's seccomp
filters anymore. You can also see this from the point where we're calling
seccomp_filter_release(). It's after __exit_signal() and at this point,
tsk->sighand will already have been NULLed which is required for
thread-sync and filter installation alike.

Cc: Tycho Andersen <tycho at tycho.ws>
Cc: Kees Cook <keescook at chromium.org>
Cc: Matt Denton <mpdenton at google.com>
Cc: Sargun Dhillon <sargun at sargun.me>
Cc: Jann Horn <jannh at google.com>
Cc: Chris Palmer <palmer at google.com>
Cc: Aleksa Sarai <cyphar at cyphar.com>
Cc: Robert Sesek <rsesek at google.com>
Cc: Jeffrey Vander Stoep <jeffv at google.com>
Cc: Linux Containers <containers at lists.linux-foundation.org>
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
/* v2 */
patch not present

/* v3 */
patch introduced
---
 include/linux/seccomp.h |  4 +--
 kernel/exit.c           |  1 +
 kernel/fork.c           |  1 -
 kernel/seccomp.c        | 60 ++++++++++++++++++++++++-----------------
 4 files changed, 38 insertions(+), 28 deletions(-)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index 4192369b8418..1644922af19a 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -82,10 +82,10 @@ static inline int seccomp_mode(struct seccomp *s)
 #endif /* CONFIG_SECCOMP */
 
 #ifdef CONFIG_SECCOMP_FILTER
-extern void put_seccomp_filter(struct task_struct *tsk);
+extern void seccomp_filter_release(struct task_struct *tsk);
 extern void get_seccomp_filter(struct task_struct *tsk);
 #else  /* CONFIG_SECCOMP_FILTER */
-static inline void put_seccomp_filter(struct task_struct *tsk)
+static inline void seccomp_filter_release(struct task_struct *tsk)
 {
 	return;
 }
diff --git a/kernel/exit.c b/kernel/exit.c
index ce2a75bc0ade..5490cc04f436 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -218,6 +218,7 @@ void release_task(struct task_struct *p)
 	}
 
 	write_unlock_irq(&tasklist_lock);
+	seccomp_filter_release(p);
 	proc_flush_pid(thread_pid);
 	put_pid(thread_pid);
 	release_thread(p);
diff --git a/kernel/fork.c b/kernel/fork.c
index 48ed22774efa..b948a14da94f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -471,7 +471,6 @@ void free_task(struct task_struct *tsk)
 #endif
 	rt_mutex_debug_task_free(tsk);
 	ftrace_graph_exit_task(tsk);
-	put_seccomp_filter(tsk);
 	arch_release_task_struct(tsk);
 	if (tsk->flags & PF_KTHREAD)
 		free_kthread_struct(tsk);
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 0ba2d6d0800f..55251b1fe03f 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -367,6 +367,40 @@ static inline pid_t seccomp_can_sync_threads(void)
 	return 0;
 }
 
+static inline void seccomp_filter_free(struct seccomp_filter *filter)
+{
+	if (filter) {
+		bpf_prog_destroy(filter->prog);
+		kfree(filter);
+	}
+}
+
+static void __put_seccomp_filter(struct seccomp_filter *orig)
+{
+	/* Clean up single-reference branches iteratively. */
+	while (orig && refcount_dec_and_test(&orig->refs)) {
+		struct seccomp_filter *freeme = orig;
+		orig = orig->prev;
+		seccomp_filter_free(freeme);
+	}
+}
+
+/**
+ * seccomp_filter_release - Detach the task from its filter tree
+ *			    and drop its reference count during
+ *			    exit.
+ *
+ * This function should only be called when the task is exiting as
+ * it detaches it from its filter tree.
+ */
+void seccomp_filter_release(struct task_struct *tsk)
+{
+	struct seccomp_filter *cur = tsk->seccomp.filter;
+
+	tsk->seccomp.filter = NULL;
+	__put_seccomp_filter(cur);
+}
+
 /**
  * seccomp_sync_threads: sets all threads to use current's filter
  *
@@ -396,7 +430,7 @@ static inline void seccomp_sync_threads(unsigned long flags)
 		 * current's path will hold a reference.  (This also
 		 * allows a put before the assignment.)
 		 */
-		put_seccomp_filter(thread);
+		__put_seccomp_filter(thread->seccomp.filter);
 		smp_store_release(&thread->seccomp.filter,
 				  caller->seccomp.filter);
 
@@ -567,30 +601,6 @@ void get_seccomp_filter(struct task_struct *tsk)
 	__get_seccomp_filter(orig);
 }
 
-static inline void seccomp_filter_free(struct seccomp_filter *filter)
-{
-	if (filter) {
-		bpf_prog_destroy(filter->prog);
-		kfree(filter);
-	}
-}
-
-static void __put_seccomp_filter(struct seccomp_filter *orig)
-{
-	/* Clean up single-reference branches iteratively. */
-	while (orig && refcount_dec_and_test(&orig->refs)) {
-		struct seccomp_filter *freeme = orig;
-		orig = orig->prev;
-		seccomp_filter_free(freeme);
-	}
-}
-
-/* put_seccomp_filter - decrements the ref count of tsk->seccomp.filter */
-void put_seccomp_filter(struct task_struct *tsk)
-{
-	__put_seccomp_filter(tsk->seccomp.filter);
-}
-
 static void seccomp_init_siginfo(kernel_siginfo_t *info, int syscall, int reason)
 {
 	clear_siginfo(info);
-- 
2.26.2