[PATCH 4/8] mm/huge_memory: handle buggy PMD entry in zap_huge_pmd()

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

A recent bug I analysed [0] 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() and exit indicating
failure.

[0]:https://lore.kernel.org/all/6b3d7ad7-49e1-407a-903d-3103704160d8@lucifer.local/

Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
---
 mm/huge_memory.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index bba1ba1f6b67..8e6b7ba11448 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2478,6 +2478,10 @@ 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 false;
 	}
 
 	if (folio_test_anon(folio)) {
-- 
2.53.0