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

[Brian Gerst]

On Thu, Mar 1, 2018 at 11:50 AM, Joerg Roedel <jroedel@xxxxxxx> wrote:
> On Thu, Mar 01, 2018 at 09:33:11AM -0500, Waiman Long wrote:
>> On 03/01/2018 08:34 AM, Joerg Roedel wrote:
>> I think that should fix the issue of debug exception from userspace.
>>
>> One thing that I am not certain about is whether debug exception can
>> happen even if the IF flag is cleared. If it can, debug exception should
>> be handled like NMI as the state of the CR3 can be indeterminate if the
>> exception happens in the entry/exit code.
>
> I am actually not 100% sure where it can happen, from the code it can
> happen from anywhere, except when we are running on an espfix stack.
>
> So I am not sure we need the same complex handling NMIs need wrt. to
> switching the cr3s.

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.

--
Brian Gerst