Re: [PATCH] scatterlist: Validate page before calling PageSlab()

[Alan Mikhak]

On Tue, Oct 1, 2019 at 5:16 AM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
>
> On Mon, Sep 30, 2019 at 04:22:35PM -0700, Alan Mikhak wrote:
> > From: Alan Mikhak <alan.mikhak@xxxxxxxxxx>
> >
> > Modify sg_miter_stop() to validate the page pointer
> > before calling PageSlab(). This check prevents a crash
> > that will occur if PageSlab() gets called with a page
> > pointer that is not backed by page struct.
> >
> > A virtual address obtained from ioremap() for a physical
> > address in PCI address space can be assigned to a
> > scatterlist segment using the public scatterlist API
> > as in the following example:
> >
> > my_sg_set_page(struct scatterlist *sg,
> >                const void __iomem *ioaddr,
> >                size_t iosize)
> > {
> >       sg_set_page(sg,
> >               virt_to_page(ioaddr),
> >               (unsigned int)iosize,
> >               offset_in_page(ioaddr));
> >       sg_init_marker(sg, 1);
> > }
> >
> > If the virtual address obtained from ioremap() is not
> > backed by a page struct, virt_to_page() returns an
> > invalid page pointer. However, sg_copy_buffer() can
> > correctly recover the original virtual address. Such
> > addresses can successfully be assigned to scatterlist
> > segments to transfer data across the PCI bus with
> > sg_copy_buffer() if it were not for the crash in
> > PageSlab() when called by sg_miter_stop().
>
> I thought we already agreed in general that putting things that don't
> have struct page into the scatter list was not allowed?
>
> Jason

Thanks Jason for your comment.

Cost of adding page structs to a large PCI I/O address range can be
quite substantial. Allowing PCI I/O pages without page structs may be
desirable. Perhaps it is worth considering this cost.

Scatterlist has no problem doing its memcpy() from pages without a
page struct that were obtained from ioremap(). It is only at
sg_miter_stop() that the call to PageSlab() prevents such use by
crashing. This seems accidental, not by design. Scatterlist API
function sg_set_page() allows any page pointer to be assigned even if
it has no page struct. It doesn't check if the page pointer is valid.
Its description doesn't say that a page struct is required.

Regards,
Alan