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

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Message-ID: <20190116054613.GA11670@nautica>
Date:   Wed, 16 Jan 2019 06:46:13 +0100
From:   Dominique Martinet <asmadeus@...ewreck.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Josh Snyder <joshs@...flix.com>,
        Dave Chinner <david@...morbit.com>,
        Jiri Kosina <jikos@...nel.org>,
        Matthew Wilcox <willy@...radead.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

Linus Torvalds wrote on Wed, Jan 16, 2019:
> *Very* few people want to run their databases as root.

In the case of happycache, this isn't the database doing the
dump/restore, but a separate process that could have the cap - it's
better if we can do without though, and from his readme he runs as user
cassandra in the /var/lib/cassandra directory for example so that'd
match the file owner.

For pgfincore, it's a postgres extension so the main process does it -
but it does have files open as write as well as being the owner.

> Jiri's original patch kind of acknowledged that by making the new test
> be conditional, and off by default. So then it's a "only do this for
> lockdown mode, because normal people won't find it acceptable".
> 
> And I'm not a huge fan of that approach. If you don't protect normal
> people, then what's the point, really?

I agree with that. 
"Being owner or has cap" (whichever cap) is probably OK.
On the other hand, writeability check makes more sense in general -
could we somehow check if the user has write access to the file instead
of checking if it currently is opened read-write?

-- 
Dominique