mm/gup: handle NULL pages in unpin_user_pages()

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

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 94efde1 ("mm/gup: avoid an unnecessary allocation call for FOLL_LONGTERM cases")
Signed-off-by: John Hubbard <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Oscar Salvador <[email protected]>
Cc: Vivek Kasireddy <[email protected]>
Cc: Dave Airlie <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Dongwon Kim <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Junxiao Chang <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: johnhubbard

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))
@@ -409,6 +414,10 @@ void unpin_user_pages(struct page **pages, unsigned long npages)
 
 	sanity_check_pinned_pages(pages, npages);
 	for (i = 0; i < npages; i += nr) {
+		if (!pages[i]) {
+			nr = 1;
+			continue;
+		}
 		folio = gup_folio_next(pages, npages, i, &nr);
 		gup_put_folio(folio, nr, FOLL_PIN);
 	}