[PATCH] io_uring/kbuf: limit legacy provided buffer lists to USHRT_MAX

[Jens Axboe <axboe@kernel.dk>]

The buffer ID for a provided buffer is an unsigned short, and hence
there can only be 64k added to any given buffer list before having
duplicate BIDs. Cap the legacy provided buffers at 64k in the list.
This is mostly to prevent silly stall reports from syzbot, which
likes to dump tons of buffers into a list and then have kernels with
lockdep and kasan churning through them and hitting long wait times
for buffer pruning at ring exit time.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/io_uring/kbuf.c b/io_uring/kbuf.c
index 8cce3ebd813f..1e123ea94233 100644
--- a/io_uring/kbuf.c
+++ b/io_uring/kbuf.c
@@ -108,6 +108,7 @@ bool io_kbuf_recycle_legacy(struct io_kiocb *req, unsigned issue_flags)
 	buf = req->kbuf;
 	bl = io_buffer_get_list(ctx, buf->bgid);
 	list_add(&buf->list, &bl->buf_list);
+	bl->nbufs++;
 	req->flags &= ~REQ_F_BUFFER_SELECTED;
 
 	io_ring_submit_unlock(ctx, issue_flags);
@@ -122,6 +123,7 @@ static void __user *io_provided_buffer_select(struct io_kiocb *req, size_t *len,
 
 		kbuf = list_first_entry(&bl->buf_list, struct io_buffer, list);
 		list_del(&kbuf->list);
+		bl->nbufs--;
 		if (*len == 0 || *len > kbuf->len)
 			*len = kbuf->len;
 		if (list_empty(&bl->buf_list))
@@ -390,6 +392,7 @@ static int io_remove_buffers_legacy(struct io_ring_ctx *ctx,
 	for (i = 0; i < nbufs && !list_empty(&bl->buf_list); i++) {
 		nxt = list_first_entry(&bl->buf_list, struct io_buffer, list);
 		list_del(&nxt->list);
+		bl->nbufs--;
 		kfree(nxt);
 		cond_resched();
 	}
@@ -491,14 +494,24 @@ static int io_add_buffers(struct io_ring_ctx *ctx, struct io_provide_buf *pbuf,
 {
 	struct io_buffer *buf;
 	u64 addr = pbuf->addr;
-	int i, bid = pbuf->bid;
+	int ret = -ENOMEM, i, bid = pbuf->bid;
 
 	for (i = 0; i < pbuf->nbufs; i++) {
+		/*
+		 * Nonsensical to have more than sizeof(bid) buffers in a
+		 * buffer list, as the application then has no way of knowing
+		 * which duplicate bid refers to what buffer.
+		 */
+		if (bl->nbufs == USHRT_MAX) {
+			ret = -EOVERFLOW;
+			break;
+		}
 		buf = kmalloc(sizeof(*buf), GFP_KERNEL_ACCOUNT);
 		if (!buf)
 			break;
 
 		list_add_tail(&buf->list, &bl->buf_list);
+		bl->nbufs++;
 		buf->addr = addr;
 		buf->len = min_t(__u32, pbuf->len, MAX_RW_COUNT);
 		buf->bid = bid;
diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
index 4d2c209d1a41..5d83c7adc739 100644
--- a/io_uring/kbuf.h
+++ b/io_uring/kbuf.h
@@ -21,6 +21,9 @@ struct io_buffer_list {
 		struct list_head buf_list;
 		struct io_uring_buf_ring *buf_ring;
 	};
+	/* count of classic/legacy buffers in buffer list */
+	int nbufs;
+
 	__u16 bgid;
 
 	/* below is for ring provided buffers */

-- 
Jens Axboe