mm: Force TLB flush for PFNMAP mappings before unlink_file_vma()

commit b67fbebd4c upstream.

Some drivers rely on having all VMAs through which a PFN might be
accessible listed in the rmap for correctness.
However, on X86, it was possible for a VMA with stale TLB entries
to not be listed in the rmap.

This was fixed in mainline with
commit b67fbebd4c ("mmu_gather: Force tlb-flush VM_PFNMAP vmas"),
but that commit relies on preceding refactoring in
commit 18ba064e42 ("mmu_gather: Let there be one tlb_{start,end}_vma()
implementation") and commit 1e9fdf21a4 ("mmu_gather: Remove per arch
tlb_{start,end}_vma()").

This patch provides equivalent protection without needing that
refactoring, by forcing a TLB flush between removing PTEs in
unmap_vmas() and the call to unlink_file_vma() in free_pgtables().

[This is a stable-specific rewrite of the upstream commit!]
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/mmap.c

@@ -2572,6 +2572,18 @@ static void unmap_region(struct mm_struct *mm,
 	tlb_gather_mmu(&tlb, mm, start, end);
 	update_hiwater_rss(mm);
 	unmap_vmas(&tlb, vma, start, end);
+
+	/*
+	 * Ensure we have no stale TLB entries by the time this mapping is
+	 * removed from the rmap.
+	 * Note that we don't have to worry about nested flushes here because
+	 * we're holding the mm semaphore for removing the mapping - so any
+	 * concurrent flush in this region has to be coming through the rmap,
+	 * and we synchronize against that using the rmap lock.
+	 */
+	if ((vma->vm_flags & (VM_PFNMAP|VM_MIXEDMAP)) != 0)
+		tlb_flush_mmu(&tlb);
+
 	free_pgtables(&tlb, vma, prev ? prev->vm_end : FIRST_USER_ADDRESS,
 				 next ? next->vm_start : USER_PGTABLES_CEILING);
 	tlb_finish_mmu(&tlb, start, end);