linux-kernel - Re: [PATCH] mm/mincore: allow for making sys

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Message-ID: <20190110145020.GW6310@bombadil.infradead.org>
Date:   Thu, 10 Jan 2019 06:50:20 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Chinner <david@...morbit.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>, Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

On Thu, Jan 10, 2019 at 11:44:24AM +1100, Dave Chinner wrote:
> And, really, this would be just another band-aid over a symptom of
> the information leak - it doesn't prevent users from being able to
> control page cache invalidation. It just removes one method, just
> like hacking mincore only removes one method of observing the page
> cache.  And, like mincore(), there's every chance it impacts on
> userspace in a negative manner and so we need to be very careful
> here.

Putting the mincore() / cache timing information leak aside though,
the current behaviour of XFS means that an attacker can screw up the
performance of random applications just by repeatedly doing O_DIRECT
reads of libc.so.

Maybe O_DIRECT reads should be forbidden from files on XFS unless you
also have write access to them?  (eg owner).