Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points

[Linus Torvalds]

On Thu, Mar 1, 2018 at 10:24 AM, Brian Gerst <brgerst@xxxxxxxxx> wrote:
>
> The IF flag only affects external maskable interrupts, not traps or
> faults.  You do need to check CR3 because SYSENTER does not clear TF
> and will immediately cause a debug trap on kernel entry (with user
> CR3) if set.  That is why the code existed before to check for the
> entry stack for debug/NMI.

Note that debug traps can happen regardless of TF, Think kgdb etc.
Arguably kgdb users get what they deserve, but still.. I think root
can set kernel breakpoints too.

          Linus