mm/memory.c: fix potential pte_unmap_unlock pte error

Since commit 42e4089 ("x86/speculation/l1tf: Disallow non privileged
high MMIO PROT_NONE mappings"), when the first pfn modify is not allowed,
we would break the loop with pte unchanged.  Then the wrong pte - 1 would
be passed to pte_unmap_unlock.

Andi said:

 "While the fix is correct, I'm not sure if it actually is a real bug.
  Is there any architecture that would do something else than unlocking
  the underlying page? If it's just the underlying page then it should
  be always the same page, so no bug"

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 42e4089 ("x86/speculation/l1tf: Disallow non privileged high MMIO PROT_NONE mappings")
Signed-off-by: Hongxiang Lou <[email protected]>
Signed-off-by: Miaohe Lin <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

mm/memory.c

@@ -2177,11 +2177,11 @@ static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
 			unsigned long addr, unsigned long end,
 			unsigned long pfn, pgprot_t prot)
 {
-	pte_t *pte;
+	pte_t *pte, *mapped_pte;
 	spinlock_t *ptl;
 	int err = 0;
 
-	pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
+	mapped_pte = pte = pte_alloc_map_lock(mm, pmd, addr, &ptl);
 	if (!pte)
 		return -ENOMEM;
 	arch_enter_lazy_mmu_mode();
@@ -2195,7 +2195,7 @@ static int remap_pte_range(struct mm_struct *mm, pmd_t *pmd,
 		pfn++;
 	} while (pte++, addr += PAGE_SIZE, addr != end);
 	arch_leave_lazy_mmu_mode();
-	pte_unmap_unlock(pte - 1, ptl);
+	pte_unmap_unlock(mapped_pte, ptl);
 	return err;
 }