mm: Don't pin ZERO_PAGE in pin_user_pages()

Make pin_user_pages*() leave a ZERO_PAGE unpinned if it extracts a pointer
to it from the page tables and make unpin_user_page*() correspondingly
ignore a ZERO_PAGE when unpinning.  We don't want to risk overrunning a
zero page's refcount as we're only allowed ~2 million pins on it -
something that userspace can conceivably trigger.

Add a pair of functions to test whether a page or a folio is a ZERO_PAGE.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Christoph Hellwig <hch@infradead.org>
cc: David Hildenbrand <david@redhat.com>
cc: Lorenzo Stoakes <lstoakes@gmail.com>
cc: Andrew Morton <akpm@linux-foundation.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: Matthew Wilcox <willy@infradead.org>
cc: Jan Kara <jack@suse.cz>
cc: Jeff Layton <jlayton@kernel.org>
cc: Jason Gunthorpe <jgg@nvidia.com>
cc: Logan Gunthorpe <logang@deltatee.com>
cc: Hillf Danton <hdanton@sina.com>
cc: Christian Brauner <brauner@kernel.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>
cc: linux-fsdevel@vger.kernel.org
cc: linux-block@vger.kernel.org
cc: linux-kernel@vger.kernel.org
cc: linux-mm@kvack.org
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20230526214142.958751-2-dhowells@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 30 insertions, 1 deletions

diff --git a/mm/gup.c b/mm/gup.c
index bbe416236593..ad28261dcafd 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -51,7 +51,8 @@ static inline void sanity_check_pinned_pages(struct page **pages,
 		struct page *page = *pages;
 		struct folio *folio = page_folio(page);
 
-		if (!folio_test_anon(folio))
+		if (is_zero_page(page) ||
+		    !folio_test_anon(folio))
 			continue;
 		if (!folio_test_large(folio) || folio_test_hugetlb(folio))
 			VM_BUG_ON_PAGE(!PageAnonExclusive(&folio->page), page);
@@ -132,6 +133,13 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
 		struct folio *folio;
 
 		/*
+		 * Don't take a pin on the zero page - it's not going anywhere
+		 * and it is used in a *lot* of places.
+		 */
+		if (is_zero_page(page))
+			return page_folio(page);
+
+		/*
 		 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
 		 * right zone, so fail and let the caller fall back to the slow
 		 * path.
@@ -180,6 +188,8 @@ struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
 static void gup_put_folio(struct folio *folio, int refs, unsigned int flags)
 {
 	if (flags & FOLL_PIN) {
+		if (is_zero_folio(folio))
+			return;
 		node_stat_mod_folio(folio, NR_FOLL_PIN_RELEASED, refs);
 		if (folio_test_large(folio))
 			atomic_sub(refs, &folio->_pincount);
@@ -225,6 +235,13 @@ int __must_check try_grab_page(struct page *page, unsigned int flags)
 		folio_ref_inc(folio);
 	else if (flags & FOLL_PIN) {
 		/*
+		 * Don't take a pin on the zero page - it's not going anywhere
+		 * and it is used in a *lot* of places.
+		 */
+		if (is_zero_page(page))
+			return 0;
+
+		/*
 		 * Similar to try_grab_folio(): be sure to *also*
 		 * increment the normal page refcount field at least once,
 		 * so that the page really is pinned.
@@ -3079,6 +3096,9 @@ EXPORT_SYMBOL_GPL(get_user_pages_fast);
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for further details.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page() will not remove pins from it.
  */
 int pin_user_pages_fast(unsigned long start, int nr_pages,
 			unsigned int gup_flags, struct page **pages)
@@ -3110,6 +3130,9 @@ EXPORT_SYMBOL_GPL(pin_user_pages_fast);
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for details.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page*() will not remove pins from it.
  */
 long pin_user_pages_remote(struct mm_struct *mm,
 			   unsigned long start, unsigned long nr_pages,
@@ -3143,6 +3166,9 @@ EXPORT_SYMBOL(pin_user_pages_remote);
  *
  * FOLL_PIN means that the pages must be released via unpin_user_page(). Please
  * see Documentation/core-api/pin_user_pages.rst for details.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page*() will not remove pins from it.
  */
 long pin_user_pages(unsigned long start, unsigned long nr_pages,
 		    unsigned int gup_flags, struct page **pages,
@@ -3161,6 +3187,9 @@ EXPORT_SYMBOL(pin_user_pages);
  * pin_user_pages_unlocked() is the FOLL_PIN variant of
  * get_user_pages_unlocked(). Behavior is the same, except that this one sets
  * FOLL_PIN and rejects FOLL_GET.
+ *
+ * Note that if a zero_page is amongst the returned pages, it will not have
+ * pins in it and unpin_user_page*() will not remove pins from it.
  */
 long pin_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
 			     struct page **pages, unsigned int gup_flags)