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
Signed-off-by: Jann Horn <jannh@google.com>
---
 fs/btrfs/send.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index 3b929f0e8f04..4e36550618e5 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -8158,7 +8158,7 @@ long btrfs_ioctl_send(struct inode *inode, struct btrfs_ioctl_send_args *arg)
 	}
 
 	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;
 	}

base-commit: 98b1cc82c4affc16f5598d4fa14b1858671b2263
-- 
2.43.0.rc1.413.gea7ed67945-goog

On Fri, Nov 24, 2023 at 05:48:31PM +0100, Jann Horn wrote:
> 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
> Signed-off-by: Jann Horn <jannh@google.com>

Added to misc-next, thanks.