userfaultfd: fix move_pages_pte() splitting folio under RCU read lock

While testing the split PMD path with lockdep enabled I've got an "Invalid
wait context" error caused by split_huge_page_to_list() trying to lock
anon_vma->rwsem while inside RCU read section.  The issues is due to
move_pages_pte() calling split_folio() under RCU read lock.  Fix this by
unmapping the PTEs and exiting RCU read section before splitting the folio
and then retrying.  The same retry pattern is used when locking the folio
or anon_vma in this function.  After splitting the large folio we unlock
and release it because after the split the old folio might not be the one
that contains the src_addr.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Suren Baghdasaryan <[email protected]>
Reviewed-by: Peter Xu <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Axel Rasmussen <[email protected]>
Cc: Brian Geffon <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Kalesh Singh <[email protected]>
Cc: Liam R. Howlett <[email protected]>
Cc: Lokesh Gidra <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Rapoport (IBM) <[email protected]>
Cc: Nicolas Geoffray <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: ZhangPeng <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 9 insertions, 0 deletions

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 5e718014e671..216ab4c8621f 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1078,9 +1078,18 @@ retry:
 
 		/* at this point we have src_folio locked */
 		if (folio_test_large(src_folio)) {
+			/* split_folio() can block */
+			pte_unmap(&orig_src_pte);
+			pte_unmap(&orig_dst_pte);
+			src_pte = dst_pte = NULL;
 			err = split_folio(src_folio);
 			if (err)
 				goto out;
+			/* have to reacquire the folio after it got split */
+			folio_unlock(src_folio);
+			folio_put(src_folio);
+			src_folio = NULL;
+			goto retry;
 		}
 
 		if (!src_anon_vma) {