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

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Message-ID: <nycvar.YFH.7.76.1901052108390.16954@cbobk.fhfr.pm>
Date:   Sat, 5 Jan 2019 21:12:22 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>, linux-mm@...ck.org,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

On Sat, 5 Jan 2019, Linus Torvalds wrote:

> > There are possibilities [1] how mincore() could be used as a converyor of
> > a sidechannel information about pagecache metadata.
> 
> Can we please just limit it to vma's that are either anonymous, or map
> a file that the user actually owns?
> 
> Then the capability check could be for "override the file owner check"
> instead, which makes tons of sense.

Makes sense. 

I am still not completely sure what to return in such cases though; we can 
either blatantly lie and always pretend that the pages are resident (to 
avoid calling process entering some prefaulting mode), or return -ENOMEM 
for mappings of files that don't belong to the user (in case it's not 
CAP_SYS_ADMIN one).

-- 
Jiri Kosina
SUSE Labs