linux-kernel - Re: [PATCH] vfio/pci: make the vfio_pci_mmap

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Message-ID: <20210309234127.GM2356281@nvidia.com>
Date:   Tue, 9 Mar 2021 19:41:27 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>,
        "Zengtao (B)" <prime.zeng@...ilicon.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Michel Lespinasse <walken@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Max Gurtovoy <mgurtovoy@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linuxarm <linuxarm@...wei.com>
Subject: Re: [PATCH] vfio/pci: make the vfio_pci_mmap_fault reentrant

On Tue, Mar 09, 2021 at 12:26:07PM -0700, Alex Williamson wrote:

> In the new series, I think the fault handler becomes (untested):
> 
> static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
> {
>         struct vm_area_struct *vma = vmf->vma;
>         struct vfio_pci_device *vdev = vma->vm_private_data;
>         unsigned long base_pfn, pgoff;
>         vm_fault_t ret = VM_FAULT_SIGBUS;
> 
>         if (vfio_pci_bar_vma_to_pfn(vma, &base_pfn))
>                 return ret;
> 
>         pgoff = (vmf->address - vma->vm_start) >> PAGE_SHIFT;

I don't think this math is completely safe, it needs to parse the
vm_pgoff..

I'm worried userspace could split/punch/mangle a VMA using
munmap/mremap/etc/etc in a way that does update the pg_off but is
incompatible with the above.

Jason