linux-kernel - Re: [PATCH 1/2] arm64: mm: abort uaccess retries upon fatal signal

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Message-ID: <596659FF.7070605@arm.com>
Date:   Wed, 12 Jul 2017 18:18:55 +0100
From:   James Morse <james.morse@....com>
To:     Mark Rutland <mark.rutland@....com>, linux-arch@...r.kernel.org
CC:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        catalin.marinas@....com, labbott@...hat.com, linux@...linux.org.uk,
        stable@...r.kernel.org, steve.capper@....com, will.deacon@....com,
        viro@...iv.linux.org.uk, peterz@...radead.org, luto@...capital.net
Subject: Re: [PATCH 1/2] arm64: mm: abort uaccess retries upon fatal signal

Hi Mark,

On 11/07/17 15:19, Mark Rutland wrote:
> When there's a fatal signal pending, arm64's do_page_fault()
> implementation returns 0. The intent is that we'll return to the
> faulting userspace instruction, delivering the signal on the way.
> 
> However, if we take a fatal signal during fixing up a uaccess, this
> results in a return to the faulting kernel instruction, which will be
> instantly retried, resulting in the same fault being taken forever. As
> the task never reaches userspace, the signal is not delivered, and the
> task is left unkillable. While the task is stuck in this state, it can
> inhibit the forward progress of the system.
> 
> To avoid this, we must ensure that when a fatal signal is pending, we
> apply any necessary fixup for a faulting kernel instruction. Thus we
> will return to an error path, and it is up to that code to make forward
> progress towards delivering the fatal signal.

VM_FAULT_RETRY's 'I released your locks' behaviour is pretty nasty, but this
looks right. FWIW:
Reviewed-by: James Morse <james.morse@....com>

I also gave this a spin through LTP on Juno, based on v4.12-defconfig:
Tested-by: James Morse <james.morse@....com>


Thanks,

James