linux-kernel - Re: [patch] mm: vmap fix overflow

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Message-Id: <20090211182050.24425324.akpm@linux-foundation.org>
Date:	Wed, 11 Feb 2009 18:20:50 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
Cc:	hch@...radead.org, linux-kernel@...r.kernel.org, stable@...nel.org
Subject: Re: [patch] mm: vmap fix overflow

On Thu, 12 Feb 2009 02:39:31 +0100 Nick Piggin <npiggin@...e.de> wrote:

> On Tue, Feb 10, 2009 at 02:44:20PM -0800, Andrew Morton wrote:
> > On Tue, 10 Feb 2009 06:51:19 +0100
> > Nick Piggin <npiggin@...e.de> wrote:
> > 
> > > This patch is appropriate for 2.6.28 too.
> > > 
> > > --
> > > 
> > > The new vmap allocator can wrap the address and get confused in the case of
> > > large allocations or VMALLOC_END near the end of address space.
> > > 
> > > Problem reported by Christoph Hellwig on a 32-bit XFS workload.
> > > 
> > > Signed-off-by: Nick Piggin <npiggin@...e.de>
> > > ---
> > >  mm/vmalloc.c |    6 ++++++
> > >  1 file changed, 6 insertions(+)
> > > 
> > > Index: linux-2.6/mm/vmalloc.c
> > > ===================================================================
> > > --- linux-2.6.orig/mm/vmalloc.c
> > > +++ linux-2.6/mm/vmalloc.c
> > > @@ -334,6 +334,9 @@ retry:
> > >  	addr = ALIGN(vstart, align);
> > >  
> > >  	spin_lock(&vmap_area_lock);
> > > +	if (addr + size < addr)
> > > +		goto overflow;
> > > +
> > >  	/* XXX: could have a last_hole cache */
> > >  	n = vmap_area_root.rb_node;
> > >  	if (n) {
> > > @@ -365,6 +368,8 @@ retry:
> > >  
> > >  		while (addr + size > first->va_start && addr + size <= vend) {
> > >  			addr = ALIGN(first->va_end + PAGE_SIZE, align);
> > > +			if (addr + size < addr)
> > > +				goto overflow;
> > >  
> > >  			n = rb_next(&first->rb_node);
> > >  			if (n)
> > > @@ -375,6 +380,7 @@ retry:
> > >  	}
> > >  found:
> > >  	if (addr + size > vend) {
> > > +overflow:
> > >  		spin_unlock(&vmap_area_lock);
> > >  		if (!purged) {
> > >  			purge_vmap_area_lazy();
> > 
> > well...
> > 
> > 
> > If a caller tries to allocate 0x1000 bytes at address 0xfffff000, this
> > code will think that it overflowed.  But it didn't.
> > 
> > Presumably nobody ever tries to do that, but it seems a bit sloppy?
> 
> Oh that's true, good catch. I guess that should be (addr + size - 1)?
> Because we care about the last byte that was actually allocated to us
> (inclusive, rather than exclusive).

That would work.  It wouldd give weird results for size==0, but probably
that's already checked for somewhere(?)

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