# Re: \[PATCH v4 2/9\] fs: add O\_ALLOW\_ENCODED open flag

- From: Amir Goldstein \<amir73il@gmail.com\>
- Date: 2020-02-29 10:44:51 
- List: linux-api
- Page: https://ratatoskr.run/linux-api/2020/02/174223
- Thread: https://ratatoskr.run/linux-fsdevel/2020/02/174204/t.md

```text
On Sat, Feb 29, 2020 at 1:14 AM Omar Sandoval [ref] wrote:
>
> From: Omar Sandoval <redacted>
>
> The upcoming RWF_ENCODED operation introduces some security concerns:
>
> 1. Compressed writes will pass arbitrary data to decompression
>    algorithms in the kernel.
> 2. Compressed reads can leak truncated/hole punched data.
>
> Therefore, we need to require privilege for RWF_ENCODED. It's not
> possible to do the permissions checks at the time of the read or write
> because, e.g., io_uring submits IO from a worker thread. So, add an open
> flag which requires CAP_SYS_ADMIN. It can also be set and cleared with
> fcntl(). The flag is not cleared in any way on fork or exec; it should
> probably be used with O_CLOEXEC in most cases.
>

So let's be more proactive and disallow setting O_ALLOW_ENCODED without
O_CLOEXEC, shall we?

> Note that the usual issue that unknown open flags are ignored doesn't
> really matter for O_ALLOW_ENCODED; if the kernel doesn't support
> O_ALLOW_ENCODED, then it doesn't support RWF_ENCODED, either.
>

And for that matter, setting O_ALLOW_ENCODED without O_CLOEXEC
won't do any harm with old kernels - even better, it can serve as a fast
test for kernel RWF_ENCODED support using only the openat() syscall.

Thanks,
Amir.
```