linux-kernel - Re: [PATCH] aio: fix mremap after fork null-deref

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Date:   Wed, 11 Jan 2023 15:16:38 -0500
From:   Seth Jenkins <sethjenkins@...gle.com>
To:     Jeff Moyer <jmoyer@...hat.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Benjamin LaHaise <bcrl@...ck.org>,
        linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
        linux-kernel@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        Pavel Emelyanov <xemul@...allels.com>, stable@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] aio: fix mremap after fork null-deref

> I wonder if it would be better to not copy the ring mapping on fork.
> Something like the below?  I think that would be more in line with
> expectations (the ring isn't available in the child process).

I like this idea a lot, but would this be viewed as subtly changing
the userland to kernel interface?

If we're okay with this change though, I think it makes sense.


On Wed, Jan 11, 2023 at 2:37 PM Jeff Moyer <jmoyer@...hat.com> wrote:
>
> Hi, Seth,
>
> Seth Jenkins <sethjenkins@...gle.com> writes:
>
> > Commit e4a0d3e720e7 ("aio: Make it possible to remap aio ring") introduced
> > a null-deref if mremap is called on an old aio mapping after fork as
> > mm->ioctx_table will be set to NULL.
> >
> > Fixes: e4a0d3e720e7 ("aio: Make it possible to remap aio ring")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Seth Jenkins <sethjenkins@...gle.com>
> > ---
> >  fs/aio.c | 20 +++++++++++---------
> >  1 file changed, 11 insertions(+), 9 deletions(-)
> >
> > diff --git a/fs/aio.c b/fs/aio.c
> > index 5b2ff20ad322..74eae7de7323 100644
> > --- a/fs/aio.c
> > +++ b/fs/aio.c
> > @@ -361,16 +361,18 @@ static int aio_ring_mremap(struct vm_area_struct *vma)
> >       spin_lock(&mm->ioctx_lock);
> >       rcu_read_lock();
> >       table = rcu_dereference(mm->ioctx_table);
> > -     for (i = 0; i < table->nr; i++) {
> > -             struct kioctx *ctx;
> > -
> > -             ctx = rcu_dereference(table->table[i]);
> > -             if (ctx && ctx->aio_ring_file == file) {
> > -                     if (!atomic_read(&ctx->dead)) {
> > -                             ctx->user_id = ctx->mmap_base = vma->vm_start;
> > -                             res = 0;
> > +     if (table) {
> > +             for (i = 0; i < table->nr; i++) {
> > +                     struct kioctx *ctx;
> > +
> > +                     ctx = rcu_dereference(table->table[i]);
> > +                     if (ctx && ctx->aio_ring_file == file) {
> > +                             if (!atomic_read(&ctx->dead)) {
> > +                                     ctx->user_id = ctx->mmap_base = vma->vm_start;
> > +                                     res = 0;
> > +                             }
> > +                             break;
> >                       }
> > -                     break;
> >               }
> >       }
>
> I wonder if it would be better to not copy the ring mapping on fork.
> Something like the below?  I think that would be more in line with
> expectations (the ring isn't available in the child process).
>
> -Jeff
>
> diff --git a/fs/aio.c b/fs/aio.c
> index 562916d85cba..dbf3b0749cb4 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -390,7 +390,7 @@ static const struct vm_operations_struct aio_ring_vm_ops = {
>
>  static int aio_ring_mmap(struct file *file, struct vm_area_struct *vma)
>  {
> -       vma->vm_flags |= VM_DONTEXPAND;
> +       vma->vm_flags |= VM_DONTEXPAND|VM_DONTCOPY;
>         vma->vm_ops = &aio_ring_vm_ops;
>         return 0;
>  }
>