linux-kernel - Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

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Message-ID: <nycvar.YFH.7.76.1903070047360.19912@cbobk.fhfr.pm>
Date:   Thu, 7 Mar 2019 01:01:10 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Daniel Gruss <daniel@...ss.cc>
Subject: Re: [PATCH 1/3] mm/mincore: make mincore() more conservative

On Wed, 6 Mar 2019, Andrew Morton wrote:

> > The semantics of what mincore() considers to be resident is not completely
> > clear, but Linux has always (since 2.3.52, which is when mincore() was
> > 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
> > (if it tried to) successfully open for writing.
> 
> "for writing" comes as a bit of a surprise.  Why not for reading?

I guess this is a rhetorical question from you :) but fair enough, good 
point, I'll explain this a bit more in the changelog and in the code 
comments.

> > @@ -189,8 +197,13 @@ static long do_mincore(unsigned long addr, unsigned long pages, unsigned char *v
> >  	vma = find_vma(current->mm, addr);
> >  	if (!vma || addr < vma->vm_start)
> >  		return -ENOMEM;
> > -	mincore_walk.mm = vma->vm_mm;
> >  	end = min(vma->vm_end, addr + (pages << PAGE_SHIFT));
> > +	if (!can_do_mincore(vma)) {
> > +		unsigned long pages = (end - addr) >> PAGE_SHIFT;
> 
> I'm not sure this is correct in all cases.   If
> 
> 	addr = 4095
> 	vma->vm_end = 4096
> 	pages = 1000
> 
> then `end' is 4096 and `(end - addr) << PAGE_SHIFT' is zero, but it
> should have been 1.

Good catch! It should rather be something like

	unsigned long pages = (end >> PAGE_SHIFT) - (addr >> PAGE_SHIFT);

I'll fix that up and resend tomorrow.

Thanks,

-- 
Jiri Kosina
SUSE Labs