[PATCH 5.4 467/475] io_uring: fix fs->users overflow

[Greg Kroah-Hartman]

From: Pavel Begunkov <asml.silence@xxxxxxxxx>

There is a bunch of cases where we can grab req->fs but not put it, this
can be used to cause a controllable overflow with further implications.
Release req->fs in the request free path and make sure we zero the field
to be sure we don't do it twice.

Fixes: cac68d12c531 ("io_uring: grab ->fs as part of async offload")
Reported-by: Bing-Jhong Billy Jheng <billy@xxxxxxxxxxx>
Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 fs/io_uring.c |   28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -438,6 +438,22 @@ static struct io_ring_ctx *io_ring_ctx_a
 	return ctx;
 }
 
+static void io_req_put_fs(struct io_kiocb *req)
+{
+	struct fs_struct *fs = req->fs;
+
+	if (!fs)
+		return;
+
+	spin_lock(&req->fs->lock);
+	if (--fs->users)
+		fs = NULL;
+	spin_unlock(&req->fs->lock);
+	if (fs)
+		free_fs_struct(fs);
+	req->fs = NULL;
+}
+
 static inline bool __io_sequence_defer(struct io_ring_ctx *ctx,
 				       struct io_kiocb *req)
 {
@@ -695,6 +711,7 @@ static void io_free_req_many(struct io_r
 
 static void __io_free_req(struct io_kiocb *req)
 {
+	io_req_put_fs(req);
 	if (req->file && !(req->flags & REQ_F_FIXED_FILE))
 		fput(req->file);
 	percpu_ref_put(&req->ctx->refs);
@@ -1701,16 +1718,7 @@ static int io_send_recvmsg(struct io_kio
 			ret = -EINTR;
 	}
 
-	if (req->fs) {
-		struct fs_struct *fs = req->fs;
-
-		spin_lock(&req->fs->lock);
-		if (--fs->users)
-			fs = NULL;
-		spin_unlock(&req->fs->lock);
-		if (fs)
-			free_fs_struct(fs);
-	}
+	io_req_put_fs(req);
 	io_cqring_add_event(req->ctx, sqe->user_data, ret);
 	io_put_req(req);
 	return 0;