[PATCH 1/1] mm/debug_vm_pgtable: fix alignment for pmd/pud_advanced_tests()

[Gerald Schaefer]

In pmd/pud_advanced_tests(), the vaddr is aligned up to the next pmd/pud
entry, and so it does not match the given pmdp/pudp and (aligned down) pfn
any more.

For s390, this results in memory corruption, because the IDTE instruction
used e.g. in xxx_get_and_clear() will take the vaddr for some calculations,
in combination with the given pmdp. It will then end up with a wrong table
origin, ending on ...ff8, and some of those wrongly set low-order bits will
also select a wrong pagetable level for the index addition. IDTE could
therefore invalidate (or 0x20) something outside of the page tables,
depending on the wrongly picked index, which in turn depends on the random
vaddr.

As result, we sometimes see "BUG task_struct (Not tainted): Padding
overwritten" on s390, where one 0x5a padding value got overwritten with
0x7a.

Fix this by aligning down, similar to how the pmd/pud_aligned pfns are
calculated.

Fixes: a5c3b9ffb0f40 ("mm/debug_vm_pgtable: add tests validating advanced arch page table helpers")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.9+
Signed-off-by: Gerald Schaefer <gerald.schaefer@xxxxxxxxxxxxx>
---
 mm/debug_vm_pgtable.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
index 6ff92c8b0a00..f7b23565a04f 100644
--- a/mm/debug_vm_pgtable.c
+++ b/mm/debug_vm_pgtable.c
@@ -193,7 +193,7 @@ static void __init pmd_advanced_tests(struct mm_struct *mm,
 
 	pr_debug("Validating PMD advanced\n");
 	/* Align the address wrt HPAGE_PMD_SIZE */
-	vaddr = (vaddr & HPAGE_PMD_MASK) + HPAGE_PMD_SIZE;
+	vaddr &= HPAGE_PMD_MASK;
 
 	pgtable_trans_huge_deposit(mm, pmdp, pgtable);
 
@@ -318,7 +318,7 @@ static void __init pud_advanced_tests(struct mm_struct *mm,
 
 	pr_debug("Validating PUD advanced\n");
 	/* Align the address wrt HPAGE_PUD_SIZE */
-	vaddr = (vaddr & HPAGE_PUD_MASK) + HPAGE_PUD_SIZE;
+	vaddr &= HPAGE_PUD_MASK;
 
 	pud = pfn_pud(pfn, prot);
 	set_pud_at(mm, vaddr, pudp, pud);
-- 
2.25.1