Re: [PATCH v1] io_uring/register.c: fix NULL pointer dereference in io_register_resize_rings

[Linus Torvalds <torvalds@linuxfoundation.org>]

On Mon, 9 Mar 2026 at 11:35, Jens Axboe <axboe@kernel.dk> wrote:
>
> --- a/io_uring/register.c
> +++ b/io_uring/register.c
> @@ -575,6 +575,7 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
>          * ctx->mmap_lock as well. Likewise, hold the completion lock over the
>          * duration of the actual swap.
>          */
> +       smp_store_release(&ctx->in_resize, 1);
>         mutex_lock(&ctx->mmap_lock);
>         spin_lock(&ctx->completion_lock);

The store-release doesn't actually make sense here. It just says "this
store is visible after all previous stores".

It can still be delayed arbitraritly, and migrate down into the locked
regions, and be visible to other cpus much later.

On x86, getting a lock will be a full memory barrier, but that's not
true everywhere else: locks keep things *inside* the locked region
inside the lock, but don't stop things *outside* the locked region
from moving into it.

End result: the smp_store_release does nothing. You should use a write
barrier (or a smp_store_mb(), but that's expensive).

But even *that* won't work - because the irq can already be running on
another CPU, and maybe it already tested 'in_resize', and saw a zero,
and then did that

     atomic_or(IORING_SQ_TASKRUN, &ctx->rings->sq_flags);

afterwards.

> @@ -647,6 +648,7 @@ static int io_register_resize_rings(struct io_ring_ctx *ctx, void __user *arg)
>         if (ctx->sq_data)
>                 io_sq_thread_unpark(ctx->sq_data);
>
> +       smp_store_release(&ctx->in_resize, 0);

On the release side, the store_release would make sense - the store is
visible to others after all the other stores are done (including,
obviously, the new 'rings' calue)

But see above. This just doesn't *work*, because the irq - running on
another cpu - will do the flag test and the cts->rings access as two
separate operations.

All these semantics means that 'in_resize' needs to basically be a lock.

You can then use 'trylock()' in irq context *around* the whole
sequence of using ctx->rings, to avoid disabling interrupts.

                Linus