Re: [PATCH] fix page leak during core dump

Hugh Dickins
2007-03-30T22:02:41Z

On Fri, 30 Mar 2007, Andrew Morton wrote:
> On Thu, 29 Mar 2007 13:39:13 -0700
> Brian Pomerantz <bapper@piratehaven.org> 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@mvista.com>
> >
> > 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