On Fri, Nov 2, 2018 at 3:56 PM Kirill A. Shutemov <kirill@xxxxxxxxxxxxx> wrote: > On Fri, Nov 02, 2018 at 01:22:42PM +0000, Will Deacon wrote: > > From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > > > Commit eb66ae030829605d61fbef1909ce310e29f78821 upstream. > > I have never seen the original patch on mailing lists, so I'll reply to > the backport. For context, the original bug report is public at https://bugs.chromium.org/p/project-zero/issues/detail?id=1695 . > > This is a backport to stable 4.4.y. > > > > Jann Horn points out that our TLB flushing was subtly wrong for the > > mremap() case. What makes mremap() special is that we don't follow the > > usual "add page to list of pages to be freed, then flush tlb, and then > > free pages". No, mremap() obviously just _moves_ the page from one page > > table location to another. > > > > That matters, because mremap() thus doesn't directly control the > > lifetime of the moved page with a freelist: instead, the lifetime of the > > page is controlled by the page table locking, that serializes access to > > the entry. > > I believe we do control the lifetime of the page with mmap_sem, don't we? Nope. For file-backed pages, someone can come through the file mapping and free our pages, e.g. through the ftruncate() syscall. > I mean any shoot down of the page from a mapping would require at least > down_read(mmap_sem) and we hold down_write(mmap_sem). Hm? > > > As a result, we need to flush the TLB not just before releasing the lock > > for the source location (to avoid any concurrent accesses to the entry), > > but also before we release the destination page table lock (to avoid the > > TLB being flushed after somebody else has already done something to that > > page). > > > > This also makes the whole "need_flush" logic unnecessary, since we now > > always end up flushing the TLB for every valid entry. > > > > Reported-and-tested-by: Jann Horn <jannh@xxxxxxxxxx> > > Acked-by: Will Deacon <will.deacon@xxxxxxx> > > Tested-by: Ingo Molnar <mingo@xxxxxxxxxx> > > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > > [will: backport to 4.4 stable] > > Signed-off-by: Will Deacon <will.deacon@xxxxxxx> > > --- > > mm/huge_memory.c | 6 +++++- > > mm/mremap.c | 21 ++++++++++++++++----- > > 2 files changed, 21 insertions(+), 6 deletions(-) > > > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > > index c4ea57ee2fd1..465786cd6490 100644 > > --- a/mm/huge_memory.c > > +++ b/mm/huge_memory.c > > @@ -1511,7 +1511,7 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma, > > spinlock_t *old_ptl, *new_ptl; > > int ret = 0; > > pmd_t pmd; > > - > > + bool force_flush = false; > > struct mm_struct *mm = vma->vm_mm; > > > > if ((old_addr & ~HPAGE_PMD_MASK) || > > @@ -1539,6 +1539,8 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma, > > if (new_ptl != old_ptl) > > spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); > > pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd); > > + if (pmd_present(pmd)) > > + force_flush = true; > > VM_BUG_ON(!pmd_none(*new_pmd)); > > > > if (pmd_move_must_withdraw(new_ptl, old_ptl)) { > > @@ -1547,6 +1549,8 @@ int move_huge_pmd(struct vm_area_struct *vma, struct vm_area_struct *new_vma, > > pgtable_trans_huge_deposit(mm, new_pmd, pgtable); > > } > > set_pmd_at(mm, new_addr, new_pmd, pmd_mksoft_dirty(pmd)); > > + if (force_flush) > > + flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE); > > if (new_ptl != old_ptl) > > spin_unlock(new_ptl); > > spin_unlock(old_ptl); > > diff --git a/mm/mremap.c b/mm/mremap.c > > index fe7b7f65f4f4..450b306d473e 100644 > > --- a/mm/mremap.c > > +++ b/mm/mremap.c > > @@ -96,6 +96,8 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, > > struct mm_struct *mm = vma->vm_mm; > > pte_t *old_pte, *new_pte, pte; > > spinlock_t *old_ptl, *new_ptl; > > + bool force_flush = false; > > + unsigned long len = old_end - old_addr; > > > > /* > > * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma > > @@ -143,12 +145,26 @@ static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd, > > if (pte_none(*old_pte)) > > continue; > > pte = ptep_get_and_clear(mm, old_addr, old_pte); > > + /* > > + * If we are remapping a valid PTE, make sure > > + * to flush TLB before we drop the PTL for the PTE. > > + * > > + * NOTE! Both old and new PTL matter: the old one > > + * for racing with page_mkclean(), the new one to > > + * make sure the physical page stays valid until > > + * the TLB entry for the old mapping has been > > + * flushed. > > + */ > > Could you elaborate on the race with page_mkclean()? > > I think the new logic is unnecessary strict (and slow). > > Any barely sane userspace must not access the old mapping after > mremap(MREMAP_MAYMOVE) called and must not access the new mapping > before the mremap() returns. > > The old logic *should* be safe if this argument valid. > > Do I miss something? > > > + if (pte_present(pte)) > > + force_flush = true; > > pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr); > > pte = move_soft_dirty_pte(pte); > > set_pte_at(mm, new_addr, new_pte, pte); > > } > > > > arch_leave_lazy_mmu_mode(); > > + if (force_flush) > > + flush_tlb_range(vma, old_end - len, old_end); > > if (new_ptl != old_ptl) > > spin_unlock(new_ptl); > > pte_unmap(new_pte - 1); > > @@ -168,7 +184,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > > { > > unsigned long extent, next, old_end; > > pmd_t *old_pmd, *new_pmd; > > - bool need_flush = false; > > unsigned long mmun_start; /* For mmu_notifiers */ > > unsigned long mmun_end; /* For mmu_notifiers */ > > > > @@ -207,7 +222,6 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > > anon_vma_unlock_write(vma->anon_vma); > > } > > if (err > 0) { > > - need_flush = true; > > continue; > > } else if (!err) { > > split_huge_page_pmd(vma, old_addr, old_pmd); > > @@ -224,10 +238,7 @@ unsigned long move_page_tables(struct vm_area_struct *vma, > > extent = LATENCY_LIMIT; > > move_ptes(vma, old_pmd, old_addr, old_addr + extent, > > new_vma, new_pmd, new_addr, need_rmap_locks); > > - need_flush = true; > > } > > - if (likely(need_flush)) > > - flush_tlb_range(vma, old_end-len, old_addr); > > > > mmu_notifier_invalidate_range_end(vma->vm_mm, mmun_start, mmun_end); > > > > -- > > 2.1.4 > > > > -- > Kirill A. Shutemov
