linux-kernel - Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn

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Date:   Mon, 8 May 2023 13:48:30 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        alex.williamson@...hat.com, kevin.tian@...el.com,
        yishaih@...dia.com, shameerali.kolothum.thodi@...wei.com
Subject: Re: [PATCH] vfio/pci: take mmap write lock for io_remap_pfn_range

On Mon, May 08, 2023 at 08:58:42PM +0800, Yan Zhao wrote:
> In VFIO type1, vaddr_get_pfns() will try fault in MMIO PFNs after
> pin_user_pages_remote() returns -EFAULT.
> 
> follow_fault_pfn
>  fixup_user_fault
>   handle_mm_fault
>    handle_mm_fault
>     do_fault
>      do_shared_fault
>       do_fault
>        __do_fault
>         vfio_pci_mmap_fault
>          io_remap_pfn_range
>           remap_pfn_range
>            track_pfn_remap
>             vm_flags_set         ==> mmap_assert_write_locked(vma->vm_mm)
>            remap_pfn_range_notrack
>             vm_flags_set         ==> mmap_assert_write_locked(vma->vm_mm)
> 
> As io_remap_pfn_range() will call vm_flags_set() to update vm_flags [1],
> holding of mmap write lock is required.
> So, update vfio_pci_mmap_fault() to drop mmap read lock and take mmap
> write lock.
> 
> [1] https://lkml.kernel.org/r/20230126193752.297968-3-surenb@google.com
> commit bc292ab00f6c ("mm: introduce vma->vm_flags wrapper functions")
> commit 1c71222e5f23
> ("mm: replace vma->vm_flags direct modifications with modifier calls")
> 
> Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> ---
>  drivers/vfio/pci/vfio_pci_core.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/vfio/pci/vfio_pci_core.c b/drivers/vfio/pci/vfio_pci_core.c
> index a5ab416cf476..5082f89152b3 100644
> --- a/drivers/vfio/pci/vfio_pci_core.c
> +++ b/drivers/vfio/pci/vfio_pci_core.c
> @@ -1687,6 +1687,12 @@ static vm_fault_t vfio_pci_mmap_fault(struct vm_fault *vmf)
>  	struct vfio_pci_mmap_vma *mmap_vma;
>  	vm_fault_t ret = VM_FAULT_NOPAGE;
>  
> +	mmap_assert_locked(vma->vm_mm);
> +	mmap_read_unlock(vma->vm_mm);
> +
> +	if (mmap_write_lock_killable(vma->vm_mm))
> +		return VM_FAULT_RETRY;

Certainly not..

I'm not sure how to resolve this properly, set the flags in advance?

The address space conversion?

Jason