netdev - Re: [PATCH net-next RFC] Add NDOs for hardware timestamp get/set

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Date:   Sat, 1 Apr 2023 10:55:33 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Vladimir Oltean <vladimir.oltean@....com>
Cc:     Maxim Georgiev <glipus@...il.com>, kory.maincent@...tlin.com,
        netdev@...r.kernel.org, maxime.chevallier@...tlin.com
Subject: Re: [PATCH net-next RFC] Add NDOs for hardware timestamp get/set

On Sat, 1 Apr 2023 19:08:29 +0300 Vladimir Oltean wrote:
> > Let's refactor this differently, we need net_hwtstamp_validate()
> > to run on the same in-kernel copy as we'll send down to the driver.
> > If we copy_from_user() twice we may validate a different thing
> > than the driver will end up seeing (ToCToU).  
> 
> I'm not sure I understand this. Since net_hwtstamp_validate() already
> contains a copy_from_user() call, don't we already call copy_to_user()
> twice (the second time being in all SIOCSHWTSTAMP handlers from drivers)?

After this patch we'll be passing an in-kernel-space struct to drivers
rather than the ifr they have to copy themselves. I'm saying that we
should validate that exact copy, rather than copy, validate, copy, pass
to drivers, cause user space may change the values between the two
copies.

Unlikely to cause serious bugs but seems like a good code hygiene.

This is only for the drivers converted to the NDO, obviously, 
the legacy drivers will still have to copy themselves.