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 14:39:22 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Cc:     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

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