On 11/19/24 6:33 AM, David Hildenbrand wrote:
On 19.11.24 05:49, John Hubbard wrote: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@xxxxxxxxxx> Cc: Oscar Salvador <osalvador@xxxxxxx> Cc: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx> Cc: Dave Airlie <airlied@xxxxxxxxxx> Cc: Gerd Hoffmann <kraxel@xxxxxxxxxx> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx> Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx> Cc: Jason Gunthorpe <jgg@xxxxxxxxxx> Cc: Peter Xu <peterx@xxxxxxxxxx> Cc: Arnd Bergmann <arnd@xxxxxxxx> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx> Cc: Dongwon Kim <dongwon.kim@xxxxxxxxx> Cc: Hugh Dickins <hughd@xxxxxxxxxx> Cc: Junxiao Chang <junxiao.chang@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx> --- 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.Ouch!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; +I don't particularly enjoy returning NULL here, if we don't teach the other users of that function about that possibility. There are two other users. Also: we are not setting "ntails" to 1. I think the callers uses that as "nr" to advance npages. So the caller has to make sure to set "nr = 1" in case it sees "NULL". Alternatively ...+ 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) {... handle it here if (!pages[i]) { nr = 1; continue; } No strong opinion. But I think we should either update all callers to deal with returning NULL from this function, and set "nr = 1".
Yes, that makes sense. I'll send a v2 shortly with one or the other approach implemented. I appreciate the review feedback as always! thanks, -- John Hubbard
