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

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Date:   Wed, 13 Feb 2019 15:25:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Julien Thierry <julien.thierry@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com, hpa@...or.com,
        valentin.schneider@....com
Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called
 in unsafe region

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.