[PATCH 4.14 26/30] btrfs: send: ensure send_fd is writable

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

4.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jann Horn <jannh@google.com>

commit 0ac1d13a55eb37d398b63e6ff6db4a09a2c9128c upstream.

kernel_write() requires the caller to ensure that the file is writable.
Let's do that directly after looking up the ->send_fd.

We don't need a separate bailout path because the "out" path already
does fput() if ->send_filp is non-NULL.

This has no security impact for two reasons:

 - the ioctl requires CAP_SYS_ADMIN
 - __kernel_write() bails out on read-only files - but only since 5.8,
   see commit a01ac27be472 ("fs: check FMODE_WRITE in __kernel_write")

Reported-and-tested-by: syzbot+12e098239d20385264d3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=12e098239d20385264d3
Fixes: 31db9f7c23fb ("Btrfs: introduce BTRFS_IOC_SEND for btrfs send/receive")
CC: stable@vger.kernel.org # 4.14+
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 fs/btrfs/send.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -6680,7 +6680,7 @@ long btrfs_ioctl_send(struct file *mnt_f
 	sctx->flags = arg->flags;
 
 	sctx->send_filp = fget(arg->send_fd);
-	if (!sctx->send_filp) {
+	if (!sctx->send_filp || !(sctx->send_filp->f_mode & FMODE_WRITE)) {
 		ret = -EBADF;
 		goto out;
 	}