Re: [PATCH v4 0/7] kernel: introduce uaccess logging

[Peter Collingbourne]

On Sat, Dec 11, 2021 at 9:23 AM David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Peter Collingbourne
> > Sent: 09 December 2021 22:16
> >
> > This patch series introduces a kernel feature known as uaccess
> > logging, which allows userspace programs to be made aware of the
> > address and size of uaccesses performed by the kernel during
> > the servicing of a syscall. More details on the motivation
> > for and interface to this feature are available in the file
> > Documentation/admin-guide/uaccess-logging.rst added by the final
> > patch in the series.
>
> How does this work when get_user() and put_user() are used to
> do optimised copies?
>
> While adding checks to copy_to/from_user() is going to have
> a measurable performance impact - even if nothing is done,
> adding them to get/put_user() (and friends) is going to
> make some hot paths really slow.
>
> So maybe you could add it so KASAN test kernels, but you can't
> sensibly enable it on a production kernel.
>
> Now, it might be that you could semi-sensibly log 'data' transfers.
> But have you actually looked at all the transfers that happen
> for something like sendmsg().
> The 'user copy hardening' code already has a significant impact
> on that code (in many places).

Hi David,

Yes, I realised after I sent out my patch (and while writing test
cases for it) that it didn't cover get_user()/put_user(). I have a
patch under development that will add this coverage. I used it to run
my invalid syscall and uname benchmarks and the results were basically
the same as without the coverage.

Are you aware of any benchmarks that cover sendmsg()? I can try to
look at writing my own if not. I was also planning to write a
benchmark that uses getresuid() as this was the simplest syscall that
I could find that does multiple put_user() calls.

Peter