arm64: hugetlb: fix set_huge_pte_at() to work with all swap entries

When called with a swap entry that does not embed a PFN (e.g. 
PTE_MARKER_POISONED or PTE_MARKER_UFFD_WP), the previous implementation of
set_huge_pte_at() would either cause a BUG() to fire (if CONFIG_DEBUG_VM
is enabled) or cause a dereference of an invalid address and subsequent
panic.

arm64's huge pte implementation supports multiple huge page sizes, some of
which are implemented in the page table with multiple contiguous entries. 
So set_huge_pte_at() needs to work out how big the logical pte is, so that
it can also work out how many physical ptes (or pmds) need to be written. 
It previously did this by grabbing the folio out of the pte and querying
its size.

However, there are cases when the pte being set is actually a swap entry. 
But this also used to work fine, because for huge ptes, we only ever saw
migration entries and hwpoison entries.  And both of these types of swap
entries have a PFN embedded, so the code would grab that and everything
still worked out.

But over time, more calls to set_huge_pte_at() have been added that set
swap entry types that do not embed a PFN.  And this causes the code to go
bang.  The triggering case is for the uffd poison test, commit
99aa772 ("selftests/mm: add uffd unit test for UFFDIO_POISON"), which
causes a PTE_MARKER_POISONED swap entry to be set, coutesey of commit
8a13897 ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs") -
added in v6.5-rc7.  Although review shows that there are other call sites
that set PTE_MARKER_UFFD_WP (which also has no PFN), these don't trigger
on arm64 because arm64 doesn't support UFFD WP.

Arguably, the root cause is really due to commit 18f3962 ("mm:
hugetlb: kill set_huge_swap_pte_at()"), which aimed to simplify the
interface to the core code by removing set_huge_swap_pte_at() (which took
a page size parameter) and replacing it with calls to set_huge_pte_at()
where the size was inferred from the folio, as descibed above.  While that
commit didn't break anything at the time, it did break the interface
because it couldn't handle swap entries without PFNs.  And since then new
callers have come along which rely on this working.  But given the
brokeness is only observable after commit 8a13897 ("mm: userfaultfd:
support UFFDIO_POISON for hugetlbfs"), that one gets the Fixes tag.

Now that we have modified the set_huge_pte_at() interface to pass the huge
page size in the previous patch, we can trivially fix this issue.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 8a13897 ("mm: userfaultfd: support UFFDIO_POISON for hugetlbfs")
Signed-off-by: Ryan Roberts <[email protected]>
Reviewed-by: Axel Rasmussen <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Alexander Gordeev <[email protected]>
Cc: Alexandre Ghiti <[email protected]>
Cc: Anshuman Khandual <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Christian Borntraeger <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Gerald Schaefer <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Helge Deller <[email protected]>
Cc: "James E.J. Bottomley" <[email protected]>
Cc: Lorenzo Stoakes <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Qi Zheng <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Sven Schnelle <[email protected]>
Cc: Uladzislau Rezki (Sony) <[email protected]>
Cc: Vasily Gorbik <[email protected]>
Cc: Will Deacon <[email protected]>
Cc: <[email protected]>	[6.5+]
Signed-off-by: Andrew Morton <[email protected]>

Author: Ryan Roberts

arch/arm64/mm/hugetlbpage.c

@@ -241,13 +241,6 @@ static void clear_flush(struct mm_struct *mm,
 	flush_tlb_range(&vma, saddr, addr);
 }
 
-static inline struct folio *hugetlb_swap_entry_to_folio(swp_entry_t entry)
-{
-	VM_BUG_ON(!is_migration_entry(entry) && !is_hwpoison_entry(entry));
-
-	return page_folio(pfn_to_page(swp_offset_pfn(entry)));
-}
-
 void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
 			    pte_t *ptep, pte_t pte, unsigned long sz)
 {
@@ -257,13 +250,10 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
 	unsigned long pfn, dpfn;
 	pgprot_t hugeprot;
 
-	if (!pte_present(pte)) {
-		struct folio *folio;
-
-		folio = hugetlb_swap_entry_to_folio(pte_to_swp_entry(pte));
-		ncontig = num_contig_ptes(folio_size(folio), &pgsize);
+	ncontig = num_contig_ptes(sz, &pgsize);
 
-		for (i = 0; i < ncontig; i++, ptep++)
+	if (!pte_present(pte)) {
+		for (i = 0; i < ncontig; i++, ptep++, addr += pgsize)
 			set_pte_at(mm, addr, ptep, pte);
 		return;
 	}
@@ -273,7 +263,6 @@ void set_huge_pte_at(struct mm_struct *mm, unsigned long addr,
 		return;
 	}
 
-	ncontig = find_num_contig(mm, addr, ptep, &pgsize);
 	pfn = pte_pfn(pte);
 	dpfn = pgsize >> PAGE_SHIFT;
 	hugeprot = pte_pgprot(pte);