linux-kernel - Re: [PATCH] mm:D-cache aliasing issue in cow_user

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Message-Id: <20061019001747.7da58920.akpm@osdl.org>
Date:	Thu, 19 Oct 2006 00:17:47 -0700
From:	Andrew Morton <akpm@...l.org>
To:	David Miller <davem@...emloft.net>
Cc:	dmonakhov@...nvz.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm:D-cache aliasing issue in cow_user_page

On Thu, 19 Oct 2006 00:00:27 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> > >   1452                  if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE))
> > >   1453                          memset(kaddr, 0, PAGE_SIZE);
> > >   1454                  kunmap_atomic(kaddr, KM_USER0);
> > >   #### D-cache have to be flushed here.
> > >   #### It seems it is just forgotten.
> > > 
> > >   1455                  return;
> > >   1456                  
> > >   1457          }
> > >   1458          copy_user_highpage(dst, src, va);
> > >   #### Ok here. flush_dcache_page() called from this func if arch need it 
> > >   1459  }
> > > 
> > 
> > This page has just been allocated and is private to the caller - there can
> > be no userspace mappings of it.
> 
> Unfortunately, the kernel has just touched the page and thus there are
> active cache lines for the kernel side mapping.  When we map this into
> user space, userspace might see stale cachelines instead of the
> memset() stores.

hm.  Has it always been that way or did something change?

> Architectures typically take care of this in copy_user_page() and
> clear_user_page().  The absolutely depend upon those two routines
> being used for anonymous pages, and handle the D-cache issues there.

Only anonymous pages?  There are zillions of places where we modify
pagecache without a flush, especially against the blockdev mapping (fs
metadata).

> But this code is going outside of that scope, and therefore needs
> an explicit D-cache flush.

OK, I'll add the patch.
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