Re: [PATCH 1/7] mm/memory.c: Fix race when faulting a device private page

[Michael Ellerman <mpe@ellerman.id.au>]

Alistair Popple <apopple@nvidia.com> writes:
> When the CPU tries to access a device private page the migrate_to_ram()
> callback associated with the pgmap for the page is called. However no
> reference is taken on the faulting page. Therefore a concurrent
> migration of the device private page can free the page and possibly the
> underlying pgmap. This results in a race which can crash the kernel due
> to the migrate_to_ram() function pointer becoming invalid. It also means
> drivers can't reliably read the zone_device_data field because the page
> may have been freed with memunmap_pages().
>
> Close the race by getting a reference on the page while holding the ptl
> to ensure it has not been freed. Unfortunately the elevated reference
> count will cause the migration required to handle the fault to fail. To
> avoid this failure pass the faulting page into the migrate_vma functions
> so that if an elevated reference count is found it can be checked to see
> if it's expected or not.
>
> Signed-off-by: Alistair Popple <apopple@nvidia.com>
> ---
>  arch/powerpc/kvm/book3s_hv_uvmem.c       | 15 ++++++-----
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.c | 17 +++++++------
>  drivers/gpu/drm/amd/amdkfd/kfd_migrate.h |  2 +-
>  drivers/gpu/drm/amd/amdkfd/kfd_svm.c     | 11 +++++---
>  include/linux/migrate.h                  |  8 ++++++-
>  lib/test_hmm.c                           |  7 ++---
>  mm/memory.c                              | 16 +++++++++++-
>  mm/migrate.c                             | 34 ++++++++++++++-----------
>  mm/migrate_device.c                      | 18 +++++++++----
>  9 files changed, 87 insertions(+), 41 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
> index 5980063..d4eacf4 100644
> --- a/arch/powerpc/kvm/book3s_hv_uvmem.c
> +++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
> @@ -508,10 +508,10 @@ unsigned long kvmppc_h_svm_init_start(struct kvm *kvm)
>  static int __kvmppc_svm_page_out(struct vm_area_struct *vma,
>  		unsigned long start,
>  		unsigned long end, unsigned long page_shift,
> -		struct kvm *kvm, unsigned long gpa)
> +		struct kvm *kvm, unsigned long gpa, struct page *fault_page)
>  {
>  	unsigned long src_pfn, dst_pfn = 0;
> -	struct migrate_vma mig;
> +	struct migrate_vma mig = { 0 };
>  	struct page *dpage, *spage;
>  	struct kvmppc_uvmem_page_pvt *pvt;
>  	unsigned long pfn;
> @@ -525,6 +525,7 @@ static int __kvmppc_svm_page_out(struct vm_area_struct *vma,
>  	mig.dst = &dst_pfn;
>  	mig.pgmap_owner = &kvmppc_uvmem_pgmap;
>  	mig.flags = MIGRATE_VMA_SELECT_DEVICE_PRIVATE;
> +	mig.fault_page = fault_page;
>  
>  	/* The requested page is already paged-out, nothing to do */
>  	if (!kvmppc_gfn_is_uvmem_pfn(gpa >> page_shift, kvm, NULL))
> @@ -580,12 +581,14 @@ static int __kvmppc_svm_page_out(struct vm_area_struct *vma,
>  static inline int kvmppc_svm_page_out(struct vm_area_struct *vma,
>  				      unsigned long start, unsigned long end,
>  				      unsigned long page_shift,
> -				      struct kvm *kvm, unsigned long gpa)
> +				      struct kvm *kvm, unsigned long gpa,
> +				      struct page *fault_page)
>  {
>  	int ret;
>  
>  	mutex_lock(&kvm->arch.uvmem_lock);
> -	ret = __kvmppc_svm_page_out(vma, start, end, page_shift, kvm, gpa);
> +	ret = __kvmppc_svm_page_out(vma, start, end, page_shift, kvm, gpa,
> +				fault_page);
>  	mutex_unlock(&kvm->arch.uvmem_lock);
>  
>  	return ret;
> @@ -736,7 +739,7 @@ static int kvmppc_svm_page_in(struct vm_area_struct *vma,
>  		bool pagein)
>  {
>  	unsigned long src_pfn, dst_pfn = 0;
> -	struct migrate_vma mig;
> +	struct migrate_vma mig = { 0 };
>  	struct page *spage;
>  	unsigned long pfn;
>  	struct page *dpage;
> @@ -994,7 +997,7 @@ static vm_fault_t kvmppc_uvmem_migrate_to_ram(struct vm_fault *vmf)
>  
>  	if (kvmppc_svm_page_out(vmf->vma, vmf->address,
>  				vmf->address + PAGE_SIZE, PAGE_SHIFT,
> -				pvt->kvm, pvt->gpa))
> +				pvt->kvm, pvt->gpa, vmf->page))
>  		return VM_FAULT_SIGBUS;
>  	else
>  		return 0;

I don't have a UV test system, but as-is it doesn't even compile :)

kvmppc_svm_page_out() is called via some paths other than the
migrate_to_ram callback.

I think it's correct to just pass fault_page = NULL when it's not called
from the migrate_to_ram callback?

Incremental diff below.

cheers


diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index d4eacf410956..965c9e9e500b 100644
--- a/arch/powerpc/kvm/book3s_hv_uvmem.c
+++ b/arch/powerpc/kvm/book3s_hv_uvmem.c
@@ -637,7 +637,7 @@ void kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *slot,
 			pvt->remove_gfn = true;
 
 			if (__kvmppc_svm_page_out(vma, addr, addr + PAGE_SIZE,
-						  PAGE_SHIFT, kvm, pvt->gpa))
+						  PAGE_SHIFT, kvm, pvt->gpa, NULL))
 				pr_err("Can't page out gpa:0x%lx addr:0x%lx\n",
 				       pvt->gpa, addr);
 		} else {
@@ -1068,7 +1068,7 @@ kvmppc_h_svm_page_out(struct kvm *kvm, unsigned long gpa,
 	if (!vma || vma->vm_start > start || vma->vm_end < end)
 		goto out;
 
-	if (!kvmppc_svm_page_out(vma, start, end, page_shift, kvm, gpa))
+	if (!kvmppc_svm_page_out(vma, start, end, page_shift, kvm, gpa, NULL))
 		ret = H_SUCCESS;
 out:
 	mmap_read_unlock(kvm->mm);