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

[Jason Gunthorpe]

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