linux-kernel - Re: Sleeping in user

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Date:   Fri, 23 Nov 2018 01:57:12 -0800
From:   hpa@...or.com
To:     Julien Thierry <julien.thierry@....com>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>
CC:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        James Morse <james.morse@....com>
Subject: Re: Sleeping in user_access section

On November 23, 2018 1:27:02 AM PST, Julien Thierry <julien.thierry@....com> wrote:
>Hi,
>
>I made an attempt at implementing the 
>user_access_begin()/user_access_end() macros along with the 
>get/put_user_unsafe() for arm64 by toggling the status of PAN (more or 
>less similar to x86's STAC/CTAC).
>
>With a small mistake in my patch, we realized that directly calling 
>function that could reschedule while in a user_access section could
>lead to:
>
>- scheduling another task keeping the user_access status enabled
>despite 
>the task never calling user_access_begin()
>
>- when re-scheduling the task that was mid user_access section, 
>user_access would be disabled and the task would fault on the next 
>get/put_user_unsafe.
>
>
>This is because __switch_to does not alter the user_access status when 
>switching from next to prev (at least on arm64 we currently don't, and 
>by looking at the x86 code I don't think this is done either).
>
>
> From my understanding, this is not an issue when the task in 
>user_access mode gets scheduled out/in as a result of an interrupt as 
>PAN and EFLAGS.AC get saved/restore on exception entry/exit (at least I
>
>know it is the case for PAN, I am less sure for the x86 side).
>
>
>So, the question is, should __switch_to take care of the user_access 
>status when scheduling new tasks? Or should there be a restriction
>about 
>scheduling out a task with user_access mode enabled and maybe add a 
>warning if we can detect this?
>
>(Or did we miss something and this is not an issue on x86?)
>
>Thanks,

You should never call a sleeping function from a user_access section. It is intended for very limited regions.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.