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

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Message-ID: <nycvar.YFH.7.76.1901162238310.6626@cbobk.fhfr.pm>
Date:   Wed, 16 Jan 2019 22:41:16 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Josh Snyder <joshs@...flix.com>,
        Dave Chinner <david@...morbit.com>,
        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>,
        Cyril Hrubis <chrubis@...e.cz>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged

On Wed, 16 Jan 2019, Matthew Wilcox wrote:

> > On Thu, 17 Jan 2019, Linus Torvalds wrote:
> > > As I suggested earlier in the thread, the fix for RWF_NOWAIT might be
> > > to just move the test down to after readahead.
> 
> Your patch 3/3 just removes the test.  Am I right in thinking that it
> doesn't need to be *moved* because the existing test after !PageUptodate
> catches it?

Exactly. It just initiates read-ahead for IOCB_NOWAIT cases as well, and 
if it's actually set, it'll be handled by the !PageUpdtodate case.

> Of course, there aren't any tests for RWF_NOWAIT in xfstests.  Are there 
> any in LTP?

Not in the released version AFAIK. I've asked the LTP maintainer (in our 
internal bugzilla) to take care of this thread a few days ago, but not 
sure what came out of it. Adding him (Cyril) to CC.

> Some typos in the commit messages:
> 
> > Another aproach (checking file access permissions in order to decide
> "approach"
> 
> > Subject: [PATCH 2/3] mm/mincore: make mincore() more conservative
> > 
> > The semantics of what mincore() considers to be resident is not completely
> > clearar, but Linux has always (since 2.3.52, which is when mincore() was
> "clear"
> 
> > initially done) treated it as "page is available in page cache".
> > 
> > That's potentially a problem, as that [in]directly exposes meta-information
> > about pagecache / memory mapping state even about memory not strictly belonging
> > to the process executing the syscall, opening possibilities for sidechannel
> > attacks.
> > 
> > Change the semantics of mincore() so that it only reveals pagecache information
> > for non-anonymous mappings that belog to files that the calling process could
> "belong"

Thanks.

-- 
Jiri Kosina
SUSE Labs