Commit a566a901 authored Sep 01, 2020 by Tycho Andersen Committed by Kees Cook Sep 08, 2020

seccomp: don't leak memory when filter install races

In seccomp_set_mode_filter() with TSYNC | NEW_LISTENER, we first initialize
the listener fd, then check to see if we can actually use it later in
seccomp_may_assign_mode(), which can fail if anyone else in our thread
group has installed a filter and caused some divergence. If we can't, we
partially clean up the newly allocated file: we put the fd, put the file,
but don't actually clean up the *memory* that was allocated at
filter->notif. Let's clean that up too.

To accomplish this, let's hoist the actual "detach a notifier from a
filter" code to its own helper out of seccomp_notify_release(), so that in
case anyone adds stuff to init_listener(), they only have to add the
cleanup code in one spot. This does a bit of extra locking and such on the
failure path when the filter is not attached, but it's a slow failure path
anyway.

Fixes: 51891498 ("seccomp: allow TSYNC and USER_NOTIF together")
Reported-by: <syzbot+3ad9614a12f80994c32e@syzkaller.appspotmail.com>
Signed-off-by: Tycho Andersen <tycho@tycho.pizza>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200902014017.934315-1-tycho@tycho.pizza

Signed-off-by: Kees Cook <keescook@chromium.org>

parent d012a719

kernel/seccomp.c

+10 −3

Original line number	Diff line number	Diff line
		@@ -1109,13 +1109,12 @@ out:
		}

		#ifdef CONFIG_SECCOMP_FILTER
		static int seccomp_notify_release(struct inode inode, struct file file)
		static void seccomp_notify_detach(struct seccomp_filter *filter)
		{
		struct seccomp_filter *filter = file->private_data;
		struct seccomp_knotif *knotif;

		if (!filter)
		return 0;
		return;

		mutex_lock(&filter->notify_lock);

		@@ -1142,6 +1141,13 @@ static int seccomp_notify_release(struct inode inode, struct file file)
		kfree(filter->notif);
		filter->notif = NULL;
		mutex_unlock(&filter->notify_lock);
		}

		static int seccomp_notify_release(struct inode inode, struct file file)
		{
		struct seccomp_filter *filter = file->private_data;

		seccomp_notify_detach(filter);
		__put_seccomp_filter(filter);
		return 0;
		}
		@@ -1581,6 +1587,7 @@ out_put_fd:
		listener_f->private_data = NULL;
		fput(listener_f);
		put_unused_fd(listener);
		seccomp_notify_detach(prepared);
		} else {
		fd_install(listener, listener_f);
		ret = listener;

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