> 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@xxxxxxxxxx> wrote:
>
> Hi, Seth,
>
> Seth Jenkins <sethjenkins@xxxxxxxxxx> 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@xxxxxxxxxxxxxxx
> > Signed-off-by: Seth Jenkins <sethjenkins@xxxxxxxxxx>
> > ---
> > 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;
> }
>
