Re: [PATCH] io_uring/zcrx: don't clear master_ctx from the import path

[Pavel Begunkov]

On 7/30/26 17:27, Woraphat Khiaodaeng wrote:
> import_zcrx() attaches an existing ifq to another ring.  It never calls
> zcrx_set_ring_ctx() and so never takes the ->master_ctx reference, but
> its error path still passes @ctx to zcrx_unregister(), which clears
> ->master_ctx and drops its percpu_ref whenever ifq->master_ctx == ctx.
>
> That condition is reachable.  A ring that registers an ifq with a
> non-zero event type_mask gets ->master_ctx pointed at itself, and
> nothing stops it from exporting that ifq with ZCRX_CTRL_EXPORT and
> importing the resulting fd back into the same ring.  Failing the import
> after the refcount bumps -- an argument page mapped PROT_READ makes the
> copy_to_user() in import_zcrx() return -EFAULT -- then clears the
> ->master_ctx owned by the original registration, which is still live.
>
> Refcounts stay balanced and nothing is freed early, so there is no
> splat.  The ring silently stops receiving ZCRX_EVENT_ALLOC_FAIL and
> ZCRX_EVENT_COPY: zcrx_send_notif() returns early on a NULL
> ->master_ctx, and ->master_ctx is only ever set on a freshly allocated
> ifq, so it cannot be restored without tearing the ring down.

Yep, it effectively disables event CQEs.

Reviewed-by: Pavel Begunkov <asml.silence@xxxxxxxxx>

...> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> index 76b9b0d54af9e..fea2b27272ec2 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -808,7 +808,8 @@ static int import_zcrx(struct io_ring_ctx *ctx,
>   	scoped_guard(mutex, &ctx->mmap_lock)
>   		xa_erase(&ctx->zcrx_ctxs, id);
>   err:
> -	zcrx_unregister(ifq, ctx);
> +	/* the import path never took the ->master_ctx ref, don't drop it */

nit: I'd say "... it never set ->master_ctx ..." as it's about having
a ctx and not references, but it's not worth of respinning.

> +	zcrx_unregister(ifq, NULL);
>   	return ret;
>   }
--
Pavel Begunkov