mm/huge_memory: handle buggy PMD entry in zap_huge_pmd()

A recent bug I analysed managed to, through a bug in the userfaultfd
implementation, reach an invalid point in the zap_huge_pmd() code where
the PMD was none of:

- A non-DAX, PFN or mixed map.
- The huge zero folio
- A present PMD entry
- A softleaf entry

The code at this point calls folio_test_anon() on a known-NULL folio.
Having logic like this explicitly NULL dereference in the code is hard to
understand, and makes debugging potentially more difficult.

Add an else branch to handle this case and WARN().

No functional change intended.

Link: https://lore.kernel.org/all/6b3d7ad7-49e1-407a-903d-3103704160d8@lucifer.local/
Link: https://lkml.kernel.org/r/fcf1f6de84a2ace188b6bf103fa15dde695f1ed8.1774029655.git.ljs@kernel.org
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 65e554afdf163e8c863c87865bc1487388dbeff0..2f9aec7d4952ce8ad5561e85f272d4364fb6831f 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2462,6 +2462,10 @@ bool zap_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
 
                if (!thp_migration_supported())
                        WARN_ONCE(1, "Non present huge pmd without pmd migration enabled!");
+       } else {
+               WARN_ON_ONCE(true);
+               spin_unlock(ptl);
+               return true;
        }
 
        if (folio_test_anon(folio)) {