[PATCH] mm/gup: handle NULL pages in unpin_user_pages()

[John Hubbard <jhubbard@nvidia.com>]

The recent addition of "pofs" (pages or folios) handling to gup has a
flaw: it assumes that unpin_user_pages() handles NULL pages in the
pages** array. That's not the case, as I discovered when I ran on a new
configuration on my test machine.

Fix this by skipping NULL pages in unpin_user_pages(), just like
unpin_folios() already does.

Details: when booting on x86 with "numa=fake=2 movablecore=4G" on Linux
6.12, and running this:

    tools/testing/selftests/mm/gup_longterm

...I get the following crash:

BUG: kernel NULL pointer dereference, address: 0000000000000008
RIP: 0010:sanity_check_pinned_pages+0x3a/0x2d0
...
Call Trace:
 <TASK>
 ? __die_body+0x66/0xb0
 ? page_fault_oops+0x30c/0x3b0
 ? do_user_addr_fault+0x6c3/0x720
 ? irqentry_enter+0x34/0x60
 ? exc_page_fault+0x68/0x100
 ? asm_exc_page_fault+0x22/0x30
 ? sanity_check_pinned_pages+0x3a/0x2d0
 unpin_user_pages+0x24/0xe0
 check_and_migrate_movable_pages_or_folios+0x455/0x4b0
 __gup_longterm_locked+0x3bf/0x820
 ? mmap_read_lock_killable+0x12/0x50
 ? __pfx_mmap_read_lock_killable+0x10/0x10
 pin_user_pages+0x66/0xa0
 gup_test_ioctl+0x358/0xb20
 __se_sys_ioctl+0x6b/0xc0
 do_syscall_64+0x7b/0x150
 entry_SYSCALL_64_after_hwframe+0x76/0x7e

Fixes: 94efde1d1539 ("mm/gup: avoid an unnecessary allocation call for FOLL_LONGTERM cases")
Cc: David Hildenbrand <david@redhat.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Vivek Kasireddy <vivek.kasireddy@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dongwon Kim <dongwon.kim@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Junxiao Chang <junxiao.chang@intel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
---

Hi,

I got a nasty shock when I tried out a new test machine setup last
night--I wish I'd noticed the problem earlier! But anyway, this should
make it all better...

I've asked Greg K-H to hold off on including commit 94efde1d1539
("mm/gup: avoid an unnecessary allocation call for FOLL_LONGTERM cases")
in linux-stable (6.11.y), but if this fix-to-the-fix looks good, then
maybe both fixes can ultimately end up in stable.

thanks,
John Hubbard

 mm/gup.c | 17 +++++++++++++++--
 1 file changed, 15 insertions(+), 2 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index ad0c8922dac3..6e417502728a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -52,7 +52,12 @@ static inline void sanity_check_pinned_pages(struct page **pages,
 	 */
 	for (; npages; npages--, pages++) {
 		struct page *page = *pages;
-		struct folio *folio = page_folio(page);
+		struct folio *folio;
+
+		if (!page)
+			continue;
+
+		folio = page_folio(page);
 
 		if (is_zero_page(page) ||
 		    !folio_test_anon(folio))
@@ -248,9 +253,14 @@ static inline struct folio *gup_folio_range_next(struct page *start,
 static inline struct folio *gup_folio_next(struct page **list,
 		unsigned long npages, unsigned long i, unsigned int *ntails)
 {
-	struct folio *folio = page_folio(list[i]);
+	struct folio *folio;
 	unsigned int nr;
 
+	if (!list[i])
+		return NULL;
+
+	folio = page_folio(list[i]);
+
 	for (nr = i + 1; nr < npages; nr++) {
 		if (page_folio(list[nr]) != folio)
 			break;
@@ -410,6 +420,9 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
 	sanity_check_pinned_pages(pages, npages);
 	for (i = 0; i < npages; i += nr) {
 		folio = gup_folio_next(pages, npages, i, &nr);
+		if (!folio)
+			continue;
+
 		gup_put_folio(folio, nr, FOLL_PIN);
 	}
 }
-- 
2.47.0