4.14-stable review patch. If anyone has any objections, please let me know.
------------------
From: Jann Horn <jannh@xxxxxxxxxx>
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@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=12e098239d20385264d3
Fixes: 31db9f7c23fb ("Btrfs: introduce BTRFS_IOC_SEND for btrfs send/receive")
CC: stable@xxxxxxxxxxxxxxx # 4.14+
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Reviewed-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
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;
}
