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struct page which are no longer valid because the page has been freed.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, 13 insertions, 5 deletions

diff --git a/mm/migrate_device.c b/mm/migrate_device.c
index 5ab6ab9d2ed8..8dee38ffcda2 100644
--- a/mm/migrate_device.c
+++ b/mm/migrate_device.c
@@ -325,14 +325,14 @@ static void migrate_vma_collect(struct migrate_vma *migrate)
  * folio_migrate_mapping(), except that here we allow migration of a
  * ZONE_DEVICE page.
  */
-static bool migrate_vma_check_page(struct page *page)
+static bool migrate_vma_check_page(struct page *page, struct page *fault_page)
 {
 	/*
 	 * One extra ref because caller holds an extra reference, either from
 	 * isolate_lru_page() for a regular page, or migrate_vma_collect() for
 	 * a device page.
 	 */
-	int extra = 1;
+	int extra = 1 + (page == fault_page);
 
 	/*
 	 * FIXME support THP (transparent huge page), it is bit more complex to
@@ -405,7 +405,8 @@ static void migrate_vma_unmap(struct migrate_vma *migrate)
 		if (folio_mapped(folio))
 			try_to_migrate(folio, 0);
 
-		if (page_mapped(page) || !migrate_vma_check_page(page)) {
+		if (page_mapped(page) ||
+		    !migrate_vma_check_page(page, migrate->fault_page)) {
 			if (!is_zone_device_page(page)) {
 				get_page(page);
 				putback_lru_page(page);
@@ -517,6 +518,8 @@ int migrate_vma_setup(struct migrate_vma *args)
 		return -EINVAL;
 	if (!args->src || !args->dst)
 		return -EINVAL;
+	if (args->fault_page && !is_device_private_page(args->fault_page))
+		return -EINVAL;
 
 	memset(args->src, 0, sizeof(*args->src) * nr_pages);
 	args->cpages = 0;
@@ -747,8 +750,13 @@ void migrate_vma_pages(struct migrate_vma *migrate)
 			continue;
 		}
 
-		r = migrate_folio(mapping, page_folio(newpage),
-				page_folio(page), MIGRATE_SYNC_NO_COPY);
+		if (migrate->fault_page == page)
+			r = migrate_folio_extra(mapping, page_folio(newpage),
+						page_folio(page),
+						MIGRATE_SYNC_NO_COPY, 1);
+		else
+			r = migrate_folio(mapping, page_folio(newpage),
+					page_folio(page), MIGRATE_SYNC_NO_COPY);
 		if (r != MIGRATEPAGE_SUCCESS)
 			migrate->src[i] &= ~MIGRATE_PFN_MIGRATE;
 	}