From f28d43636d6f940e60abef4f0131119836c8ebd4 Mon Sep 17 00:00:00 2001 From: Claudio Imbrenda Date: Wed, 1 Apr 2020 21:05:56 -0700 Subject: mm/gup/writeback: add callbacks for inaccessible pages MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit With the introduction of protected KVM guests on s390 there is now a concept of inaccessible pages. These pages need to be made accessible before the host can access them. While cpu accesses will trigger a fault that can be resolved, I/O accesses will just fail. We need to add a callback into architecture code for places that will do I/O, namely when writeback is started or when a page reference is taken. This is not only to enable paging, file backing etc, it is also necessary to protect the host against a malicious user space. For example a bad QEMU could simply start direct I/O on such protected memory. We do not want userspace to be able to trigger I/O errors and thus the logic is "whenever somebody accesses that page (gup) or does I/O, make sure that this page can be accessed". When the guest tries to access that page we will wait in the page fault handler for writeback to have finished and for the page_ref to be the expected value. On s390x the function is not supposed to fail, so it is ok to use a WARN_ON on failure. If we ever need some more finegrained handling we can tackle this when we know the details. Signed-off-by: Claudio Imbrenda Signed-off-by: Andrew Morton Reviewed-by: David Hildenbrand Reviewed-by: Christian Borntraeger Reviewed-by: John Hubbard Acked-by: Will Deacon Cc: Jan Kara Cc: Matthew Wilcox Cc: Ira Weiny Cc: Jérôme Glisse Cc: Al Viro Cc: Christoph Hellwig Cc: Dan Williams Cc: Dave Chinner Cc: Jason Gunthorpe Cc: Jonathan Corbet Cc: Michal Hocko Cc: Mike Kravetz Cc: Shuah Khan Cc: Vlastimil Babka Link: http://lkml.kernel.org/r/20200306132537.783769-3-imbrenda@linux.ibm.com Signed-off-by: Linus Torvalds --- mm/page-writeback.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) (limited to 'mm/page-writeback.c') diff --git a/mm/page-writeback.c b/mm/page-writeback.c index d9c42d38ee47..7326b54ab728 100644 --- a/mm/page-writeback.c +++ b/mm/page-writeback.c @@ -2764,7 +2764,7 @@ int test_clear_page_writeback(struct page *page) int __test_set_page_writeback(struct page *page, bool keep_write) { struct address_space *mapping = page_mapping(page); - int ret; + int ret, access_ret; lock_page_memcg(page); if (mapping && mapping_use_writeback_tags(mapping)) { @@ -2807,6 +2807,13 @@ int __test_set_page_writeback(struct page *page, bool keep_write) inc_zone_page_state(page, NR_ZONE_WRITE_PENDING); } unlock_page_memcg(page); + access_ret = arch_make_page_accessible(page); + /* + * If writeback has been triggered on a page that cannot be made + * accessible, it is too late to recover here. + */ + VM_BUG_ON_PAGE(access_ret != 0, page); + return ret; } -- cgit v1.2.3