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Message-ID: <0da9689ec3bf72919114a7350584624860c49495.camel@linux.ibm.com>
Subject: Re: [PATCH v2 1/3] s390/pci: Fix s390_mmio_read/write syscall page
 fault handling
From: Niklas Schnelle <schnelle@linux.ibm.com>
To: Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Heiko Carstens
 <hca@linux.ibm.com>, Vasily Gorbik <gor@linux.ibm.com>,
        Alexander Gordeev
 <agordeev@linux.ibm.com>,
        Christian Borntraeger
 <borntraeger@linux.ibm.com>,
        Sven Schnelle <svens@linux.ibm.com>,
        Alex
 Williamson <alex.williamson@redhat.com>,
        Gerd Bayer <gbayer@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Jason Gunthorpe <jgg@ziepe.ca>
Cc: linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Date: Tue, 28 May 2024 17:12:38 +0200
In-Reply-To: <20240523-vfio_pci_mmap-v2-1-0dc6c139a4f1@linux.ibm.com>
References: <20240523-vfio_pci_mmap-v2-0-0dc6c139a4f1@linux.ibm.com>
	 <20240523-vfio_pci_mmap-v2-1-0dc6c139a4f1@linux.ibm.com>
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On Thu, 2024-05-23 at 13:10 +0200, Niklas Schnelle wrote:
> The s390 MMIO syscalls when using the classic PCI instructions do not
> cause a page fault when follow_pte() fails due to the page not being
> present. Besides being a general deficiency this breaks vfio-pci's mmap()
> handling once VFIO_PCI_MMAP gets enabled as this lazily maps on first
> access. Fix this by following a failed follow_pte() with
> fixup_user_page() and retrying the follow_pte().
>=20
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
>  arch/s390/pci/pci_mmio.c | 18 +++++++++++++-----
>  1 file changed, 13 insertions(+), 5 deletions(-)
>=20
> diff --git a/arch/s390/pci/pci_mmio.c b/arch/s390/pci/pci_mmio.c
> index a90499c087f0..217defbcb4f1 100644
> --- a/arch/s390/pci/pci_mmio.c
> +++ b/arch/s390/pci/pci_mmio.c
> @@ -170,8 +170,12 @@ SYSCALL_DEFINE3(s390_pci_mmio_write, unsigned long, =
mmio_addr,
>  		goto out_unlock_mmap;
> =20
>  	ret =3D follow_pte(vma->vm_mm, mmio_addr, &ptep, &ptl);
> -	if (ret)
> -		goto out_unlock_mmap;
> +	if (ret) {
> +		fixup_user_fault(vma->vm_mm, mmio_addr, FAULT_FLAG_WRITE, NULL);
> +		ret =3D follow_pte(vma->vm_mm, mmio_addr, &ptep, &ptl);

This needs a slight adjustment in v6.10-rc1 due to a change in
follow_pte()'s signature. The above should take "vma" directly. We
could then also use current->mm in fixup_user_fault() as seems more
common. I've already pushed this to my kernel.org repository and will
send a v3 rebased on v6.10-rc1 with just this as I'm not tracking any
other feedback.

> +		if (ret)
> +			goto out_unlock_mmap;
> +	}
> =20
>  	io_addr =3D (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
>  			(mmio_addr & ~PAGE_MASK));
> @@ -305,12 +309,16 @@ SYSCALL_DEFINE3(s390_pci_mmio_read, unsigned long, =
mmio_addr,
>  	if (!(vma->vm_flags & (VM_IO | VM_PFNMAP)))
>  		goto out_unlock_mmap;
>  	ret =3D -EACCES;
> -	if (!(vma->vm_flags & VM_WRITE))
> +	if (!(vma->vm_flags & VM_READ))
>  		goto out_unlock_mmap;
> =20
>  	ret =3D follow_pte(vma->vm_mm, mmio_addr, &ptep, &ptl);
> -	if (ret)
> -		goto out_unlock_mmap;
> +	if (ret) {
> +		fixup_user_fault(vma->vm_mm, mmio_addr, 0, NULL);
> +		ret =3D follow_pte(vma->vm_mm, mmio_addr, &ptep, &ptl);

Here as well

> +		if (ret)
> +			goto out_unlock_mmap;
> +	}
> =20
>  	io_addr =3D (void __iomem *)((pte_pfn(*ptep) << PAGE_SHIFT) |
>  			(mmio_addr & ~PAGE_MASK));
>=20