Re: [patch 1/9] fs: libfs buffered write leak fix

[Nick Piggin]

On Fri, Feb 02, 2007 at 03:52:36PM -0800, Andrew Morton wrote:
> On Mon, 29 Jan 2007 11:31:46 +0100 (CET)
> Nick Piggin <npiggin@xxxxxxx> wrote:
> 
> > simple_prepare_write and nobh_prepare_write leak uninitialised kernel data.
> 
> They do?  Under what situation?

Yes, I have at least reproduced the libfs leak.

The situation is when you write into a !uptodate page, the prepare_write
function runs SetPageUptodate *before* we have copied data in. Thus you
can read uninitialised data out of there.

SetPageUptodate must not be used (or at least used carefully) in
prepare_write. commit_write is the correct place to do this.

> > Fix the former,
> 
> How?

If doing a partial-write, simply clear the whole page and set it uptodate
(don't need to get too tricky). If doing a full-write, only set it uptodate
in the commit_write.

> > make a note of the latter. Several other filesystems seem
> > to be iffy here, too.
> 
> Please, tell us what the bug is so that others have a chance of reviewing
> and, if needed, fixing those other filesystems.
> 
> > --- linux-2.6.orig/fs/libfs.c
> > +++ linux-2.6/fs/libfs.c
> > @@ -327,32 +327,35 @@ int simple_readpage(struct file *file, s
> >  int simple_prepare_write(struct file *file, struct page *page,
> >  			unsigned from, unsigned to)
> >  {
> > -	if (!PageUptodate(page)) {
> > -		if (to - from != PAGE_CACHE_SIZE) {
> > -			void *kaddr = kmap_atomic(page, KM_USER0);
> > -			memset(kaddr, 0, from);
> > -			memset(kaddr + to, 0, PAGE_CACHE_SIZE - to);
> > -			flush_dcache_page(page);
> > -			kunmap_atomic(kaddr, KM_USER0);
> > -		}
> > +	if (PageUptodate(page))
> > +		return 0;
> > +
> > +	if (to - from != PAGE_CACHE_SIZE) {
> > +		clear_highpage(page);
> > +		flush_dcache_page(page);
> >  		SetPageUptodate(page);
> >  	}
> 
> memclear_highpage_flush() is fashionable.

Good one.

> > ===================================================================
> > --- linux-2.6.orig/fs/buffer.c
> > +++ linux-2.6/fs/buffer.c
> > @@ -2344,6 +2344,8 @@ int nobh_prepare_write(struct page *page
> >  
> >  	if (is_mapped_to_disk)
> >  		SetPageMappedToDisk(page);
> > +
> > +	/* XXX: information leak vs read(2) */
> >  	SetPageUptodate(page);
> >  
> >  	/*
> 
> That comment is too terse to be useful.

OK, similar problem here - we have brought all the buffers uptodate
that we are *not* going to write over, or partially write over, but
we can have an uninitialised hole over the region we want to write.

I think just setting page uptodate in commit_write might do the
trick? (and getting rid of the set_page_dirty there).
-
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