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

[Peter Zijlstra]

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.

SMAP {ab,re}uses the EFLAGS.AC bit.

> Consequently, taking an irq in a
> user_access_{begin,end} section and then scheduling is fine, but calling
> schedule directly within such a section is not.