Re: [PATCH 8/9] Add explicit bound checks in mm/migrate.c

KOSAKI Motohiro
2009-10-01T05:54:59Z

Hi

> On Sat, 26 Sep 2009 20:54:06 +0200
> Arjan van de Ven <arjan@infradead.org> wrote:
>
> > From: Arjan van de Ven <arjan@linux.intel.com>
> > Subject: [PATCH 8/9] Add explicit bound checks in mm/migrate.c
> > CC: akpm@linux-foundation.org
> >
> > The memory migration code has some curious copy_from_user bounds,
> > that are likely ok, but are not immediately obvious to me or to GCC.
> >
> > This patch adds a simple explicit bound check; this allows GCC
> > and me to be more assured that the copy_from_user will never overwrite
> > its destination buffer.
>
> I don't really see what's being fixed here. The original code seems
> straightforward and safe enough?

I think original code is safe too.

> The identifier `chunk_nr' is a bit ambiguous. Is it "number of chunks" or
> is it "index of this chunk"?

chunk_nr is batch size. (ie it's number of chunks)

Plus, I have a review comment.

>
> >
> > diff --git a/mm/migrate.c b/mm/migrate.c
> > index 1a4bf48..5b9ebc5 100644
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -1044,11 +1044,15 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
> > int err;
> >
> > for (i = 0; i < nr_pages; i += chunk_nr) {
> > + unsigned int copy;
> > if (chunk_nr + i > nr_pages)
> > chunk_nr = nr_pages - i;
>
> A newline after end-of-locals is conventional.
>
> `i' and `chunk_nr' have type `unsigned long' and you're mixing that up
> with `unsigned int'.
>
> > - err = copy_from_user(chunk_pages, &pages[i],
> > - chunk_nr * sizeof(*chunk_pages));
>
> And we mix it up with size_t as well.
>
> The type choices are a bit confused and sloppy. Converting it all to
> `unsigned int' should be OK.
>
> > + copy = chunk_nr * sizeof(*chunk_pages);
> > + if (copy > DO_PAGES_STAT_CHUNK_NR)
> > + return -EFAULT;

this seems a bit strange.
the unit of copy is byte. but the unit of DO_PAGES_STAT_CHUNK_NR is
not byte.

> > +
> > + err = copy_from_user(chunk_pages, &pages[i], copy);
> > if (err) {
> > err = -EFAULT;
> > goto out;
>
>