linux-kernel - Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame Ownership (XPFO)

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Date:   Tue, 14 Nov 2017 19:44:30 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Tycho Andersen <tycho@...ker.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, kernel-hardening@...ts.openwall.com,
        Marco Benatto <marco.antonio.780@...il.com>,
        Juerg Haefliger <juerg.haefliger@...onical.com>, x86@...nel.org
Subject: Re: [PATCH v6 03/11] mm, x86: Add support for eXclusive Page Frame
 Ownership (XPFO)

On Mon, Nov 13, 2017 at 02:46:25PM -0800, Dave Hansen wrote:
> On 11/13/2017 02:20 PM, Dave Hansen wrote:
> > On 11/09/2017 05:09 PM, Tycho Andersen wrote:
> >> which I guess is from the additional flags in grow_dev_page() somewhere down
> >> the stack. Anyway... it seems this is a kernel allocation that's using
> >> MIGRATE_MOVABLE, so perhaps we need some more fine tuned heuristic than just
> >> all MOVABLE allocations are un-mapped via xpfo, and all the others are mapped.
> >>
> >> Do you have any ideas?
> > 
> > It still has to do a kmap() or kmap_atomic() to be able to access it.  I
> > thought you hooked into that.  Why isn't that path getting hit for these?
> 
> Oh, this looks to be accessing data mapped by a buffer_head.  It
> (rudely) accesses data via:
> 
> void set_bh_page(struct buffer_head *bh,
> ...
> 	bh->b_data = page_address(page) + offset;

We don't need to kmap in order to access MOVABLE allocations.  kmap is
only needed for HIGHMEM allocations.  So there's nothing wrong with ext4
or set_bh_page().