linux-kernel - Re: [PATCH 10/13] PCI: revoke mappings like devmem

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Message-ID: <CAPcyv4hBL68A7CZa+YnooufDH2tevoxrx32DTJMQ6OHRnec7QQ@mail.gmail.com>
Date:   Wed, 7 Oct 2020 12:33:06 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     DRI Development <dri-devel@...ts.freedesktop.org>,
        LKML <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>, Linux MM <linux-mm@...ck.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        "Linux-media@...r.kernel.org" <linux-media@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Daniel Vetter <daniel.vetter@...el.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Jan Kara <jack@...e.cz>, Bjorn Helgaas <bhelgaas@...gle.com>,
        linux-pci@...r.kernel.org
Subject: Re: [PATCH 10/13] PCI: revoke mappings like devmem

On Wed, Oct 7, 2020 at 11:11 AM Daniel Vetter <daniel.vetter@...ll.ch> wrote:
>
> Since 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims
> the region") /dev/kmem zaps ptes when the kernel requests exclusive
> acccess to an iomem region. And with CONFIG_IO_STRICT_DEVMEM, this is
> the default for all driver uses.
>
> Except there's two more ways to access pci bars: sysfs and proc mmap
> support. Let's plug that hole.

Ooh, yes, lets.

>
> For revoke_devmem() to work we need to link our vma into the same
> address_space, with consistent vma->vm_pgoff. ->pgoff is already
> adjusted, because that's how (io_)remap_pfn_range works, but for the
> mapping we need to adjust vma->vm_file->f_mapping. Usually that's done
> at ->open time, but that's a bit tricky here with all the entry points
> and arch code. So instead create a fake file and adjust vma->vm_file.

I don't think you want to share the devmem inode for this, this should
be based off the sysfs inode which I believe there is already only one
instance per resource. In contrast /dev/mem can have multiple inodes
because anyone can just mknod a new character device file, the same
problem does not exist for sysfs.