Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

Julien Thierry
2019-02-13T14:39:30Z

Hi Peter,

On 13/02/2019 14:25, Peter Zijlstra wrote:
> On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:
>> The difference is because getting preempted in the sequence above is
>> triggered off the back of an interrupt. On arm64, and I think also on x86,
>> the user access state (SMAP or PAN) is saved and restored across exceptions
>> but not across context switch.
>
> A quick reading of the SDM seems to suggest the SMAP state is part of
> EFLAGS, which is context switched just fine AFAIK.
>
I fail to see where this is happening when looking at the switch_to()
logic in x86_64.

And Peter A. didn't seem to suggest that this transfer of the eflags was
happening without them being saved on the stack through exception handling.

What am I missing?

Thanks,

--
Julien Thierry