mm: vmalloc must set pte via arch code

Patch series "Fixes for pte encapsulation bypasses", v3.

A series to improve the encapsulation of pte entries by disallowing
non-arch code from directly dereferencing pte_t pointers.


This patch (of 4):

It is bad practice to directly set pte entries within a pte table. 
Instead all modifications must go through arch-provided helpers such as
set_pte_at() to give the arch code visibility and allow it to check (and
potentially modify) the operation.

Link: https://lkml.kernel.org/r/20230602092949.545577-1-ryan.roberts@arm.com
Link: https://lkml.kernel.org/r/20230602092949.545577-2-ryan.roberts@arm.com
Fixes: 3e9a9e25 ("mm: add a vmap_pfn function")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: SeongJae Park <sj@kernel.org>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmalloc.c

@@ -2944,10 +2944,16 @@ struct vmap_pfn_data {
 static int vmap_pfn_apply(pte_t *pte, unsigned long addr, void *private)
 {
 	struct vmap_pfn_data *data = private;
+	unsigned long pfn = data->pfns[data->idx];
+	pte_t ptent;
 
-	if (WARN_ON_ONCE(pfn_valid(data->pfns[data->idx])))
+	if (WARN_ON_ONCE(pfn_valid(pfn)))
 		return -EINVAL;
-	*pte = pte_mkspecial(pfn_pte(data->pfns[data->idx++], data->prot));
+
+	ptent = pte_mkspecial(pfn_pte(pfn, data->prot));
+	set_pte_at(&init_mm, addr, pte, ptent);
+
+	data->idx++;
 	return 0;
 }