btrfs: send: ensure send_fd is writable

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: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=12e098239d20385264d3
Fixes: 31db9f7c23fb ("Btrfs: introduce BTRFS_IOC_SEND for btrfs send/receive")
CC: [e-mail] # 4.14+
Signed-off-by: Jann Horn <[e-mail]>
Reviewed-by: David Sterba <[e-mail]>
Signed-off-by: David Sterba <[e-mail]>

fs/btrfs/send.c

···
 	}
 
 	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;
 	}