linux-kernel - Re: [PATCH 2/2] mm, oom: fix potential data corruption when oom

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Message-Id: <201708111128.FEE39036.HFVSQFOtOMLFJO@I-love.SAKURA.ne.jp>
Date:   Fri, 11 Aug 2017 11:28:52 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:     mhocko@...nel.org, akpm@...ux-foundation.org
Cc:     andrea@...nel.org, kirill@...temov.name, oleg@...hat.com,
        wenwei.tww@...baba-inc.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, mhocko@...e.com
Subject: Re: [PATCH 2/2] mm, oom: fix potential data corruption when oom_reaper races with writer

Michal Hocko wrote:
> +/*
> + * Checks whether a page fault on the given mm is still reliable.
> + * This is no longer true if the oom reaper started to reap the
> + * address space which is reflected by MMF_UNSTABLE flag set in
> + * the mm. At that moment any !shared mapping would lose the content
> + * and could cause a memory corruption (zero pages instead of the
> + * original content).
> + *
> + * User should call this before establishing a page table entry for
> + * a !shared mapping and under the proper page table lock.
> + *
> + * Return 0 when the PF is safe VM_FAULT_SIGBUS otherwise.
> + */
> +static inline int check_stable_address_space(struct mm_struct *mm)
> +{
> +	if (unlikely(test_bit(MMF_UNSTABLE, &mm->flags)))
> +		return VM_FAULT_SIGBUS;
> +	return 0;
> +}
> +

Will you explain the mechanism why random values are written instead of zeros
so that this patch can actually fix the race problem? I consider that writing
random values (though it seems like portion of process image) instead of zeros
to a file might cause a security problem, and the patch that fixes it should be
able to be backported to stable kernels.