linux-kernel - Re: [PATCH 6/6] fuse: Verify userspace asks to requeue interrupt that we really sent

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Date:   Wed, 7 Nov 2018 14:55:34 +0100
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] fuse: Verify userspace asks to requeue interrupt that
 we really sent

On Tue, Nov 6, 2018 at 10:31 AM, Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> When queue_interrupt() is called from fuse_dev_do_write(),
> it came from userspace directly. Userspace may pass any
> request id, even the request's we have not interrupted
> (or even background's request). This patch adds sanity
> check to make kernel safe against that.

Okay, I understand this far.

> The problem is real interrupt may be queued and requeued
> by two tasks on two cpus. This case, the requeuer has not
> guarantees it sees FR_INTERRUPTED bit on its cpu (since
> we know nothing about the way userspace manages requests
> between its threads and whether it uses smp barriers).

This sounds like BS.   There's an explicit  smp_mb__after_atomic()
after the set_bit(FR_INTERRUPTED,...).  Which means FR_INTERRUPTED is
going to be visible on all CPU's after this, no need to fool around
with setting FR_INTERRUPTED again, etc...



>
> To eliminate this problem, queuer writes FR_INTERRUPTED
> bit again under fiq->waitq.lock, and this guarantees
> requeuer will see the bit, when checks it.
>
> I try to introduce solution, which does not affect on
> performance, and which does not force to take more
> locks. This is the reason, the below solution is worse:
>
>    request_wait_answer()
>    {
>      ...
>   +  spin_lock(&fiq->waitq.lock);
>      set_bit(FR_INTERRUPTED, &req->flags);
>   +  spin_unlock(&fiq->waitq.lock);
>      ...
>    }
>
> Also, it does not look a better idea to extend fuse_dev_do_read()
> with the fix, since it's already a big function:
>
>    fuse_dev_do_read()
>    {
>      ...
>      if (test_bit(FR_INTERRUPTED, &req->flags)) {
>   +      /* Comment */
>   +      barrier();
>   +      set_bit(FR_INTERRUPTED, &req->flags);
>          queue_interrupt(fiq, req);
>      }
>      ...
>    }
>
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> ---
>  fs/fuse/dev.c |   26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
>
> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
> index 315d395d5c02..3bfc5ed61c9a 100644
> --- a/fs/fuse/dev.c
> +++ b/fs/fuse/dev.c
> @@ -475,13 +475,27 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
>         fuse_put_request(fc, req);
>  }
>
> -static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
> +static int queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
>  {
>         bool kill = false;
>
>         if (test_bit(FR_FINISHED, &req->flags))
> -               return;
> +               return 0;
>         spin_lock(&fiq->waitq.lock);
> +       /* Check for we've sent request to interrupt this req */
> +       if (unlikely(!test_bit(FR_INTERRUPTED, &req->flags))) {
> +               spin_unlock(&fiq->waitq.lock);
> +               return -EINVAL;
> +       }
> +       /*
> +        * Interrupt may be queued from fuse_dev_do_read(), and
> +        * later requeued on other cpu by fuse_dev_do_write().
> +        * To make FR_INTERRUPTED bit visible for the requeuer
> +        * (under fiq->waitq.lock) we write it once again.
> +        */
> +       barrier();
> +       __set_bit(FR_INTERRUPTED, &req->flags);
> +
>         if (list_empty(&req->intr_entry)) {
>                 list_add_tail(&req->intr_entry, &fiq->interrupts);
>                 /*
> @@ -492,7 +506,7 @@ static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
>                 if (test_bit(FR_FINISHED, &req->flags)) {
>                         list_del_init(&req->intr_entry);
>                         spin_unlock(&fiq->waitq.lock);
> -                       return;
> +                       return 0;
>                 }
>                 wake_up_locked(&fiq->waitq);
>                 kill = true;
> @@ -500,6 +514,7 @@ static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
>         spin_unlock(&fiq->waitq.lock);
>         if (kill)
>                 kill_fasync(&fiq->fasync, SIGIO, POLL_IN);
> +       return (int)kill;
>  }
>
>  static void request_wait_answer(struct fuse_conn *fc, struct fuse_req *req)
> @@ -1959,8 +1974,9 @@ static ssize_t fuse_dev_do_write(struct fuse_dev *fud,
>                         nbytes = -EINVAL;
>                 else if (oh.error == -ENOSYS)
>                         fc->no_interrupt = 1;
> -               else if (oh.error == -EAGAIN)
> -                       queue_interrupt(&fc->iq, req);
> +               else if (oh.error == -EAGAIN &&
> +                        queue_interrupt(&fc->iq, req) < 0)
> +                       nbytes = -EINVAL;
>
>                 fuse_put_request(fc, req);
>                 fuse_copy_finish(cs);
>