userfaultfd: don't BUG_ON() if khugepaged yanks our page table

BugLink: https://bugs.launchpad.net/bugs/2085849

commit 4828d207dc5161dc7ddf9a4f6dcfd80c7dd7d20a upstream.

Since khugepaged was changed to allow retracting page tables in file
mappings without holding the mmap lock, these BUG_ON()s are wrong - get
rid of them.

We could also remove the preceding "if (unlikely(...))" block, but then we
could reach pte_offset_map_lock() with transhuge pages not just for file
mappings but also for anonymous mappings - which would probably be fine
but I think is not necessarily expected.

Link: https://lkml.kernel.org/r/20240813-uffd-thp-flip-fix-v2-2-5efa61078a41@google.com
Fixes: 1d65b771bc ("mm/khugepaged: retract_page_tables() without mmap or vma lock")
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Qi Zheng <zhengqi.arch@bytedance.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Pavel Emelyanov <xemul@virtuozzo.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
CVE-2024-46838
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Roxana Nicolescu <roxana.nicolescu@canonical.com>

mm/userfaultfd.c

@@ -717,9 +717,10 @@ retry:
 			err = -EFAULT;
 			break;
 		}
-
-		BUG_ON(pmd_none(*dst_pmd));
-		BUG_ON(pmd_trans_huge(*dst_pmd));
+		/*
+		 * For shmem mappings, khugepaged is allowed to remove page
+		 * tables under us; pte_offset_map_lock() will deal with that.
+		 */
 
 		err = mfill_atomic_pte(dst_pmd, dst_vma, dst_addr,
 				       src_addr, flags, &folio);