Date:   Fri, 18 Oct 2019 20:06:25 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/3] io_uring: add support for async work inheriting files table

On Fri, Oct 18, 2019 at 7:05 PM Jens Axboe <axboe@...nel.dk> wrote:
> On 10/18/19 10:36 AM, Jens Axboe wrote:
> >> Ignoring the locking elision, basically the logic is now this:
> >>
> >> static void io_sq_wq_submit_work(struct work_struct *work)
> >> {
> >>           struct io_kiocb *req = container_of(work, struct io_kiocb, work);
> >>           struct files_struct *cur_files = NULL, *old_files;
> >>           [...]
> >>           old_files = current->files;
> >>           [...]
> >>           do {
> >>                   struct sqe_submit *s = &req->submit;
> >>                   [...]
> >>                   if (cur_files)
> >>                           /* drop cur_files reference; borrow lifetime must
> >>                            * end before here */
> >>                           put_files_struct(cur_files);
> >>                   /* move reference ownership to cur_files */
> >>                   cur_files = s->files;
> >>                   if (cur_files) {
> >>                           task_lock(current);
> >>                           /* current->files borrows reference from cur_files;
> >>                            * existing borrow from previous loop ends here */
> >>                           current->files = cur_files;
> >>                           task_unlock(current);
> >>                   }
> >>
> >>                   [call __io_submit_sqe()]
> >>                   [...]
> >>           } while (req);
> >>           [...]
> >>           /* existing borrow ends here */
> >>           task_lock(current);
> >>           current->files = old_files;
> >>           task_unlock(current);
> >>           if (cur_files)
> >>                   /* drop cur_files reference; borrow lifetime must
> >>                    * end before here */
> >>                   put_files_struct(cur_files);
> >> }
> >>
> >> If you run two iterations of this loop, with a first element that has
> >> a ->files pointer and a second element that doesn't, then in the
> >> second run through the loop, the reference to the files_struct will be
> >> dropped while current->files still points to it; current->files is
> >> only reset after the loop has ended. If someone accesses
> >> current->files through procfs directly after that, AFAICS you'd get a
> >> use-after-free.
> >
> > Amazing how this is still broken. You are right, and it's especially
> > annoying since that's exactly the case I originally talked about (not
> > flipping current->files if we don't have to). I just did it wrong, so
> > we'll leave a dangling pointer in ->files.
> >
> > The by far most common case is if one sqe has a files it needs to
> > attach, then others that also have files will be the same set. So I want
> > to optimize for the case where we only flip current->files once when we
> > see the files, and once when we're done with the loop.
> >
> > Let me see if I can get this right...
>
> I _think_ the simplest way to do it is simply to have both cur_files and
> current->files hold a reference to the file table. That won't really add
> any extra cost as the double increments / decrements are following each
> other. Something like this incremental, totally untested.
>
>
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 2fed0badad38..b3cf3f3d7911 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -2293,9 +2293,14 @@ static void io_sq_wq_submit_work(struct work_struct *work)
>                         put_files_struct(cur_files);
>                 cur_files = s->files;
>                 if (cur_files && cur_files != current->files) {
> +                       struct files_struct *old;
> +
> +                       atomic_inc(&cur_files->count);
>                         task_lock(current);
> +                       old = current->files;
>                         current->files = cur_files;
>                         task_unlock(current);
> +                       put_files_struct(old);
>                 }
>
>                 if (!ret) {
> @@ -2390,9 +2395,13 @@ static void io_sq_wq_submit_work(struct work_struct *work)
>                 mmput(cur_mm);
>         }
>         if (old_files != current->files) {
> +               struct files_struct *old;
> +
>                 task_lock(current);
> +               old = current->files;
>                 current->files = old_files;
>                 task_unlock(current);
> +               put_files_struct(old);
>         }
>         if (cur_files)
>                 put_files_struct(cur_files);

The only part I still feel a bit twitchy about is this part at the end:

        if (old_files != current->files) {
                struct files_struct *old;

                task_lock(current);
                old = current->files;
                current->files = old_files;
                task_unlock(current);
                put_files_struct(old);
        }

If it was possible for the initial ->files to be the same as the
->files of a submission, and we got two submissions with first a
different files_struct and then our old one, then this branch would
not be executed even though it should, which would leave the refcount
of the files_struct one too high. But that probably can't happen?
Since kernel workers should be running with &init_files (I think?) and
that thing is never used for userspace tasks. But still, I'd feel
better if you could change it like this:

diff --git a/fs/io_uring.c b/fs/io_uring.c
index f9f5c70564f0..7673035d6bfe 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2265,6 +2265,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
 {
        struct io_kiocb *req = container_of(work, struct io_kiocb, work);
        struct files_struct *cur_files = NULL, *old_files;
+       bool restore_current_files = false;
        struct io_ring_ctx *ctx = req->ctx;
        struct mm_struct *cur_mm = NULL;
        struct async_list *async_list;
@@ -2313,6 +2314,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
                        current->files = cur_files;
                        task_unlock(current);
                        put_files_struct(old);
+                       restore_current_files = true;
                }

                if (!ret) {
@@ -2406,7 +2408,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
                unuse_mm(cur_mm);
                mmput(cur_mm);
        }
-       if (old_files != current->files) {
+       if (restore_current_files) {
                struct files_struct *old;

                task_lock(current);


But actually, by the way: Is this whole files_struct thing creating a
reference loop? The files_struct has a reference to the uring file,
and the uring file has ACCEPT work that has a reference to the
files_struct. If the task gets killed and the accept work blocks, the
entire files_struct will stay alive, right?

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