[PATCH 4.19 37/56] mm: Force TLB flush for PFNMAP mappings before unlink_file_vma()

Greg Kroah-Hartman
2022-09-02T12:35:29Z

From: Jann Horn <jannh@google.com>

commit b67fbebd4cf980aecbcc750e1462128bffe8ae15 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 b67fbebd4cf9 ("mmu_gather: Force tlb-flush VM_PFNMAP vmas"),
but that commit relies on preceding refactoring in
commit 18ba064e42df3 ("mmu_gather: Let there be one tlb_{start,end}_vma()
implementation") and commit 1e9fdf21a4339 ("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 | 12 ++++++++++++
1 file changed, 12 insertions(+)

--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -2572,6 +2572,18 @@ static void unmap_region(struct mm_struc
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);