Re: [PATCH v2] mm/gup: handle NULL pages in unpin_user_pages()

[David Hildenbrand <david@redhat.com>]

On 21.11.24 04:49, 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@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>
> ---

Thanks!

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb