[PATCHv2] arm64: mm: Fix VM_BUG_ON(mm != &init_mm) for trans_pgd

[Pingfan Liu]

trans_pgd_create_copy() can hit "VM_BUG_ON(mm != &init_mm)" in the
function pmd_populate_kernel().

This is the combined consequence of commit 5de59884ac0e ("arm64:
trans_pgd: pass NULL instead of init_mm to *_populate functions"), which
replaced &init_mm with NULL and commit 59511cfd08f3 ("arm64: mm: use XN
table mapping attributes for user/kernel mappings"), which introduced
the VM_BUG_ON.

Since the former sounds reasonable, it is better to work on the later.
>From the perspective of trans_pgd, two groups of functions are
considered in the later one:

  pmd_populate_kernel()
    mm == NULL should be fixed, else it hits VM_BUG_ON()
  p?d_populate()
    mm == NULL means PXN, that is OK, since trans_pgd only copies a
    linear map, no execution will happen on the map.

So it is good enough to just relax VM_BUG_ON() to disregard mm == NULL

Fixes: 59511cfd08f3 ("arm64: mm: use XN table mapping attributes for user/kernel mappings")
Signed-off-by: Pingfan Liu <kernelfans at gmail.com>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Ard Biesheuvel <ardb at kernel.org>
Cc: Will Deacon <will at kernel.org>
Cc: Pasha Tatashin <pasha.tatashin at soleen.com>
Cc: James Morse <james.morse at arm.com>
Cc: Matthias Brugger <mbrugger at suse.com>
To: linux-arm-kernel at lists.infradead.org
---
 arch/arm64/include/asm/pgalloc.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/pgalloc.h b/arch/arm64/include/asm/pgalloc.h
index 8433a2058eb1..237224484d0f 100644
--- a/arch/arm64/include/asm/pgalloc.h
+++ b/arch/arm64/include/asm/pgalloc.h
@@ -76,7 +76,7 @@ static inline void __pmd_populate(pmd_t *pmdp, phys_addr_t ptep,
 static inline void
 pmd_populate_kernel(struct mm_struct *mm, pmd_t *pmdp, pte_t *ptep)
 {
-	VM_BUG_ON(mm != &init_mm);
+	VM_BUG_ON(mm && mm != &init_mm);
 	__pmd_populate(pmdp, __pa(ptep), PMD_TYPE_TABLE | PMD_TABLE_UXN);
 }
 
-- 
2.31.1