Re: [PATCH 5.12] io_uring: Convert personality_idr to XArray

[Pavel Begunkov <asml.silence@gmail.com>]

On 08/03/2021 14:16, Pavel Begunkov wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
> 
> You can't call idr_remove() from within a idr_for_each() callback,
> but you can call xa_erase() from an xa_for_each() loop, so switch the
> entire personality_idr from the IDR to the XArray.  This manifests as a
> use-after-free as idr_for_each() attempts to walk the rest of the node
> after removing the last entry from it.

yangerkun, can you test it and similarly take care of buffer idr?


> 
> Fixes: 071698e13ac6 ("io_uring: allow registering credentials")
> Cc: stable@vger.kernel.org # 5.6+
> Reported-by: yangerkun <yangerkun@huawei.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> [Pavel: rebased (creds load was moved into io_init_req())]
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  fs/io_uring.c | 47 ++++++++++++++++++++++++-----------------------
>  1 file changed, 24 insertions(+), 23 deletions(-)
> 
> p.s. passes liburing tests well
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index 5ef9f836cccc..5505e19f1391 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -407,7 +407,8 @@ struct io_ring_ctx {
>  
>  	struct idr		io_buffer_idr;
>  
> -	struct idr		personality_idr;
> +	struct xarray		personalities;
> +	u32			pers_next;
>  
>  	struct {
>  		unsigned		cached_cq_tail;
> @@ -1138,7 +1139,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
>  	init_completion(&ctx->ref_comp);
>  	init_completion(&ctx->sq_thread_comp);
>  	idr_init(&ctx->io_buffer_idr);
> -	idr_init(&ctx->personality_idr);
> +	xa_init_flags(&ctx->personalities, XA_FLAGS_ALLOC1);
>  	mutex_init(&ctx->uring_lock);
>  	init_waitqueue_head(&ctx->wait);
>  	spin_lock_init(&ctx->completion_lock);
> @@ -6338,7 +6339,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
>  	req->work.list.next = NULL;
>  	personality = READ_ONCE(sqe->personality);
>  	if (personality) {
> -		req->work.creds = idr_find(&ctx->personality_idr, personality);
> +		req->work.creds = xa_load(&ctx->personalities, personality);
>  		if (!req->work.creds)
>  			return -EINVAL;
>  		get_cred(req->work.creds);
> @@ -8359,7 +8360,6 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
>  	mutex_unlock(&ctx->uring_lock);
>  	io_eventfd_unregister(ctx);
>  	io_destroy_buffers(ctx);
> -	idr_destroy(&ctx->personality_idr);
>  
>  #if defined(CONFIG_UNIX)
>  	if (ctx->ring_sock) {
> @@ -8424,7 +8424,7 @@ static int io_unregister_personality(struct io_ring_ctx *ctx, unsigned id)
>  {
>  	const struct cred *creds;
>  
> -	creds = idr_remove(&ctx->personality_idr, id);
> +	creds = xa_erase(&ctx->personalities, id);
>  	if (creds) {
>  		put_cred(creds);
>  		return 0;
> @@ -8433,14 +8433,6 @@ static int io_unregister_personality(struct io_ring_ctx *ctx, unsigned id)
>  	return -EINVAL;
>  }
>  
> -static int io_remove_personalities(int id, void *p, void *data)
> -{
> -	struct io_ring_ctx *ctx = data;
> -
> -	io_unregister_personality(ctx, id);
> -	return 0;
> -}
> -
>  static bool io_run_ctx_fallback(struct io_ring_ctx *ctx)
>  {
>  	struct callback_head *work, *next;
> @@ -8530,13 +8522,17 @@ static void io_ring_exit_work(struct work_struct *work)
>  
>  static void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
>  {
> +	unsigned long index;
> +	struct creds *creds;
> +
>  	mutex_lock(&ctx->uring_lock);
>  	percpu_ref_kill(&ctx->refs);
>  	/* if force is set, the ring is going away. always drop after that */
>  	ctx->cq_overflow_flushed = 1;
>  	if (ctx->rings)
>  		__io_cqring_overflow_flush(ctx, true, NULL, NULL);
> -	idr_for_each(&ctx->personality_idr, io_remove_personalities, ctx);
> +	xa_for_each(&ctx->personalities, index, creds)
> +		io_unregister_personality(ctx, index);
>  	mutex_unlock(&ctx->uring_lock);
>  
>  	io_kill_timeouts(ctx, NULL, NULL);
> @@ -9166,10 +9162,9 @@ SYSCALL_DEFINE6(io_uring_enter, unsigned int, fd, u32, to_submit,
>  }
>  
>  #ifdef CONFIG_PROC_FS
> -static int io_uring_show_cred(int id, void *p, void *data)
> +static int io_uring_show_cred(struct seq_file *m, unsigned int id,
> +		const struct cred *cred)
>  {
> -	const struct cred *cred = p;
> -	struct seq_file *m = data;
>  	struct user_namespace *uns = seq_user_ns(m);
>  	struct group_info *gi;
>  	kernel_cap_t cap;
> @@ -9237,9 +9232,13 @@ static void __io_uring_show_fdinfo(struct io_ring_ctx *ctx, struct seq_file *m)
>  		seq_printf(m, "%5u: 0x%llx/%u\n", i, buf->ubuf,
>  						(unsigned int) buf->len);
>  	}
> -	if (has_lock && !idr_is_empty(&ctx->personality_idr)) {
> +	if (has_lock && !xa_empty(&ctx->personalities)) {
> +		unsigned long index;
> +		const struct cred *cred;
> +
>  		seq_printf(m, "Personalities:\n");
> -		idr_for_each(&ctx->personality_idr, io_uring_show_cred, m);
> +		xa_for_each(&ctx->personalities, index, cred)
> +			io_uring_show_cred(m, index, cred);
>  	}
>  	seq_printf(m, "PollList:\n");
>  	spin_lock_irq(&ctx->completion_lock);
> @@ -9568,14 +9567,16 @@ static int io_probe(struct io_ring_ctx *ctx, void __user *arg, unsigned nr_args)
>  static int io_register_personality(struct io_ring_ctx *ctx)
>  {
>  	const struct cred *creds;
> +	u32 id;
>  	int ret;
>  
>  	creds = get_current_cred();
>  
> -	ret = idr_alloc_cyclic(&ctx->personality_idr, (void *) creds, 1,
> -				USHRT_MAX, GFP_KERNEL);
> -	if (ret < 0)
> -		put_cred(creds);
> +	ret = xa_alloc_cyclic(&ctx->personalities, &id, (void *)creds,
> +			XA_LIMIT(0, USHRT_MAX), &ctx->pers_next, GFP_KERNEL);
> +	if (!ret)
> +		return id;
> +	put_cred(creds);
>  	return ret;
>  }
>  
> 

-- 
Pavel Begunkov