linux-kernel - Re: KASAN: use-after-free Read in idr_for

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Date:   Sun, 29 Nov 2020 12:26:00 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     syzbot <syzbot+12056a09a0311d758e60@...kaller.appspotmail.com>,
        axboe@...nel.dk, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: KASAN: use-after-free Read in idr_for_each (2)

On Sun, Nov 29, 2020 at 07:34:29PM +0800, Hillf Danton wrote:
> >  radix_tree_next_slot include/linux/radix-tree.h:422 [inline]
> >  idr_for_each+0x206/0x220 lib/idr.c:202
> >  io_destroy_buffers fs/io_uring.c:8275 [inline]
> 
> Matthew, can you shed any light on the link between the use of idr
> routines and the UAF reported?

I presume it's some misuse of IDR by io_uring.  I'd rather io_uring
didn't use the IDR at all.  This compiles; I promise no more than that.

diff --git a/fs/io_uring.c b/fs/io_uring.c
index ef3cd7fe4416..2fcf196bb3c3 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -344,7 +344,7 @@ struct io_ring_ctx {
 	struct socket		*ring_sock;
 #endif
 
-	struct idr		io_buffer_idr;
+	struct xarray		io_buffers;
 
 	struct idr		personality_idr;
 
@@ -1298,7 +1298,7 @@ static struct io_ring_ctx *io_ring_ctx_alloc(struct io_uring_params *p)
 	INIT_LIST_HEAD(&ctx->cq_overflow_list);
 	init_completion(&ctx->ref_comp);
 	init_completion(&ctx->sq_thread_comp);
-	idr_init(&ctx->io_buffer_idr);
+	xa_init(&ctx->io_buffers);
 	idr_init(&ctx->personality_idr);
 	mutex_init(&ctx->uring_lock);
 	init_waitqueue_head(&ctx->wait);
@@ -3042,7 +3042,7 @@ static struct io_buffer *io_buffer_select(struct io_kiocb *req, size_t *len,
 
 	lockdep_assert_held(&req->ctx->uring_lock);
 
-	head = idr_find(&req->ctx->io_buffer_idr, bgid);
+	head = xa_load(&req->ctx->io_buffers, bgid);
 	if (head) {
 		if (!list_empty(&head->list)) {
 			kbuf = list_last_entry(&head->list, struct io_buffer,
@@ -3050,7 +3050,7 @@ static struct io_buffer *io_buffer_select(struct io_kiocb *req, size_t *len,
 			list_del(&kbuf->list);
 		} else {
 			kbuf = head;
-			idr_remove(&req->ctx->io_buffer_idr, bgid);
+			xa_erase(&req->ctx->io_buffers, bgid);
 		}
 		if (*len > kbuf->len)
 			*len = kbuf->len;
@@ -4130,7 +4130,8 @@ static int __io_remove_buffers(struct io_ring_ctx *ctx, struct io_buffer *buf,
 	}
 	i++;
 	kfree(buf);
-	idr_remove(&ctx->io_buffer_idr, bgid);
+	if (nbufs != -1U)
+		xa_erase(&ctx->io_buffers, bgid);
 
 	return i;
 }
@@ -4148,7 +4149,7 @@ static int io_remove_buffers(struct io_kiocb *req, bool force_nonblock,
 	lockdep_assert_held(&ctx->uring_lock);
 
 	ret = -ENOENT;
-	head = idr_find(&ctx->io_buffer_idr, p->bgid);
+	head = xa_load(&ctx->io_buffers, p->bgid);
 	if (head)
 		ret = __io_remove_buffers(ctx, head, p->bgid, p->nbufs);
 
@@ -4225,15 +4226,15 @@ static int io_provide_buffers(struct io_kiocb *req, bool force_nonblock,
 
 	lockdep_assert_held(&ctx->uring_lock);
 
-	list = head = idr_find(&ctx->io_buffer_idr, p->bgid);
+	list = head = xa_load(&ctx->io_buffers, p->bgid);
 
 	ret = io_add_buffers(p, &head);
 	if (ret < 0)
 		goto out;
 
 	if (!list) {
-		ret = idr_alloc(&ctx->io_buffer_idr, head, p->bgid, p->bgid + 1,
-					GFP_KERNEL);
+		ret = xa_err(xa_store(&ctx->io_buffers, p->bgid, head,
+					GFP_KERNEL));
 		if (ret < 0) {
 			__io_remove_buffers(ctx, head, p->bgid, -1U);
 			goto out;
@@ -8468,19 +8469,15 @@ static int io_eventfd_unregister(struct io_ring_ctx *ctx)
 	return -ENXIO;
 }
 
-static int __io_destroy_buffers(int id, void *p, void *data)
-{
-	struct io_ring_ctx *ctx = data;
-	struct io_buffer *buf = p;
-
-	__io_remove_buffers(ctx, buf, id, -1U);
-	return 0;
-}
-
 static void io_destroy_buffers(struct io_ring_ctx *ctx)
 {
-	idr_for_each(&ctx->io_buffer_idr, __io_destroy_buffers, ctx);
-	idr_destroy(&ctx->io_buffer_idr);
+	unsigned long pgid;
+	struct io_buffer *buf;
+
+	xa_for_each(&ctx->io_buffers, pgid, buf) {
+		xa_erase(&ctx->io_buffers, pgid);
+		__io_remove_buffers(ctx, buf, pgid, -1U);
+	}
 }
 
 static void io_ring_ctx_free(struct io_ring_ctx *ctx)