mm/memory.c: fix race when faulting a device private page

Patch series "Fix several device private page reference counting issues",
v2

This series aims to fix a number of page reference counting issues in
drivers dealing with device private ZONE_DEVICE pages.  These result in
use-after-free type bugs, either from accessing a struct page which no
longer exists because it has been removed or accessing fields within the
struct page which are no longer valid because the page has been freed.

During normal usage it is unlikely these will cause any problems.  However
without these fixes it is possible to crash the kernel from userspace. 
These crashes can be triggered either by unloading the kernel module or
unbinding the device from the driver prior to a userspace task exiting. 
In modules such as Nouveau it is also possible to trigger some of these
issues by explicitly closing the device file-descriptor prior to the task
exiting and then accessing device private memory.

This involves some minor changes to both PowerPC and AMD GPU code. 
Unfortunately I lack hardware to test either of those so any help there
would be appreciated.  The changes mimic what is done in for both Nouveau
and hmm-tests though so I doubt they will cause problems.


This patch (of 8):

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.

[mpe@ellerman.id.au: fix build]
  Link: https://lkml.kernel.org/r/87fsgbf3gh.fsf@mpe.ellerman.id.au
Link: https://lkml.kernel.org/r/cover.60659b549d8509ddecafad4f498ee7f03bb23c69.1664366292.git-series.apopple@nvidia.com
Link: https://lkml.kernel.org/r/d3e813178a59e565e8d78d9b9a4e2562f6494f90.1664366292.git-series.apopple@nvidia.com
Signed-off-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Lyude Paul <lyude@redhat.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Alex Sierra <alex.sierra@amd.com>
Cc: Ben Skeggs <bskeggs@redhat.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 11 insertions, 8 deletions

diff --git a/arch/powerpc/kvm/book3s_hv_uvmem.c b/arch/powerpc/kvm/book3s_hv_uvmem.c
index 598006301620..965c9e9e500b 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 @@ out_finalize:
 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;
@@ -634,7 +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 {
@@ -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;
@@ -1065,7 +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);