Re: [PATCH] seccomp: fix UAF in user-trap code

[Kees Cook <keescook@chromium.org>]

On Sat, Jan 12, 2019 at 10:24 AM Tycho Andersen <tycho@tycho.ws> wrote:
>
> On the failure path, we do an fput() of the listener fd if the filter fails
> to install (e.g. because of a TSYNC race that's lost, or if the thread is
> killed, etc.). fput() doesn't actually release the fd, it just ads it to a
> work queue. Then the thread proceeds to free the filter, even though the
> listener struct file has a reference to it.
>
> To fix this, on the failure path let's set the private data to null, so we
> know in ->release() to ignore the filter.
>
> Reported-by: syzbot+981c26489b2d1c6316ba@syzkaller.appspotmail.com
> Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> Signed-off-by: Tycho Andersen <tycho@tycho.ws>
> ---
> This is a little ugly, but I can't really think of a better way (other than
> force freeing, but the fput function that does the actual work is declared
> static with four underscores :).

This makes sense to me. Thanks for fixing it!

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

James, can you add this to your fixes tree for sending to Linus?

-Kees

> ---
>  kernel/seccomp.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index d7f538847b84..e815781ed751 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -976,6 +976,9 @@ static int seccomp_notify_release(struct inode *inode, struct file *file)
>         struct seccomp_filter *filter = file->private_data;
>         struct seccomp_knotif *knotif;
>
> +       if (!filter)
> +               return 0;
> +
>         mutex_lock(&filter->notify_lock);
>
>         /*
> @@ -1300,6 +1303,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  out_put_fd:
>         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
>                 if (ret < 0) {
> +                       listener_f->private_data = NULL;
>                         fput(listener_f);
>                         put_unused_fd(listener);
>                 } else {
> --
> 2.19.1
>


-- 
Kees Cook