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

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Message-ID: <20191001121623.GA22532@ziepe.ca>
Date:   Tue, 1 Oct 2019 09:16:23 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alan Mikhak <alan.mikhak@...ive.com>
Cc:     linux-kernel@...r.kernel.org, martin.petersen@...cle.com,
        alexios.zavras@...el.com, ming.lei@...hat.com,
        gregkh@...uxfoundation.org, tglx@...utronix.de,
        christophe.leroy@....fr, palmer@...ive.com,
        paul.walmsley@...ive.com
Subject: Re: [PATCH] scatterlist: Validate page before calling PageSlab()

On Mon, Sep 30, 2019 at 04:22:35PM -0700, Alan Mikhak wrote:
> From: Alan Mikhak <alan.mikhak@...ive.com>
> 
> 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