Re: [PATCH v4 2/9] fs: add O_ALLOW_ENCODED open flag

On Sat, Feb 29, 2020 at 1:14 AM Omar Sandoval [off-list 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.