Re: [PATCH] fix page leak during core dump

[Hugh Dickins]

On Fri, 30 Mar 2007, Andrew Morton wrote:
> On Thu, 29 Mar 2007 13:39:13 -0700
> Brian Pomerantz <bapper@xxxxxxxxxxxxxxx> wrote:
> 
> > When the dump cannot occur most likely because of a full file system
> > and the page to be written is the zero page, the call to
> > page_cache_release() is missed.
> > 
> > Signed-off-by: Brian Pomerantz <bapper@xxxxxxxxxx>
> > 
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index a2fceba..9cc4f0a 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -1704,7 +1704,10 @@ static int elf_core_dump(long signr, struct pt_regs *regs, struct file *file)
> >  				DUMP_SEEK(PAGE_SIZE);
> >  			} else {
> >  				if (page == ZERO_PAGE(addr)) {
> > -					DUMP_SEEK(PAGE_SIZE);
> > +					if (!dump_seek(file, PAGE_SIZE)) {
> > +						page_cache_release(page);
> > +						goto end_coredump;
> > +					}
> 
> Oh for gawds sake I wish we could be rid of those idiotic macros :(
> 
> This patch looks OK to me, although a refcount leak on the ZERO_PAGE is
> special, because that page is PageReserved().
> 
> It used to be the case that we'd ignore attempts to change the refcount on
> reserved pages (or at least on the ZERO_PAGE), but we changed that, so we
> now actually refcount the ZERO_PAGE.  (I think, from a quick read of the
> code.  This contradicts my memory of how it works).
> 
> So I expect the net effect here is that a sufficiently determined attacker
> can overflow the ZERO_PAGE's refcount, thus causing it to be "freed".  The
> page allocator won't actually free the page due to PG_Reserved, but it'll
> all become very noisy.
> 
> Nick, Hugh: agree?

I think so - lots of "Bad page state" messages as the count bounces
around the 0 mark, but not actually freed.  But when CONFIG_DEBUG_VM
you'll get BUG_ONs.  And I can't swear bad things won't happen some-
where once the count wraps to negative.  Easier to fix than work out
the consequences.

(Of course, Nick is right now proposing a patch to take us back the
other way, back to not accounting the ZERO_PAGE: so the fix needs
to go in, then he'll need to reverse that again in his patch.)

Doesn't fs/binfmt_elf_fdpic.c need the same fix?  It looks slightly
different there, but I think when you look closer there's exactly
the same issue?

Hugh
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