From 4647706ebeee6e50f7b9f922b095f4ec94d581c3 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Wed, 6 Sep 2017 16:21:05 -0700 Subject: [PATCH] mm: always flush VMA ranges affected by zap_page_range Nadav Amit report zap_page_range only specifies that the caller protect the VMA list but does not specify whether it is held for read or write with callers using either. madvise holds mmap_sem for read meaning that a parallel zap operation can unmap PTEs which are then potentially skipped by madvise which potentially returns with stale TLB entries present. While the API could be extended, it would be a difficult API to use. This patch causes zap_page_range() to always consider flushing the full affected range. For small ranges or sparsely populated mappings, this may result in one additional spurious TLB flush. For larger ranges, it is possible that the TLB has already been flushed and the overhead is negligible. Either way, this approach is safer overall and avoids stale entries being present when madvise returns. This can be illustrated with the following program provided by Nadav Amit and slightly modified. With the patch applied, it has an exit code of 0 indicating a stale TLB entry did not leak to userspace. ---8<--- volatile int sync_step = 0; volatile char *p; static inline unsigned long rdtsc() { unsigned long hi, lo; __asm__ __volatile__ ("rdtsc" : "=a"(lo), "=d"(hi)); return lo | (hi << 32); } static inline void wait_rdtsc(unsigned long cycles) { unsigned long tsc = rdtsc(); while (rdtsc() - tsc < cycles); } void *big_madvise_thread(void *ign) { sync_step = 1; while (sync_step != 2); madvise((void*)p, PAGE_SIZE * N_PAGES, MADV_DONTNEED); } int main(void) { pthread_t aux_thread; p = mmap(0, PAGE_SIZE * N_PAGES, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0); memset((void*)p, 8, PAGE_SIZE * N_PAGES); pthread_create(&aux_thread, NULL, big_madvise_thread, NULL); while (sync_step != 1); *p = 8; // Cache in TLB sync_step = 2; wait_rdtsc(100000); madvise((void*)p, PAGE_SIZE, MADV_DONTNEED); printf("data: %d (%s)\n", *p, (*p == 8 ? "stale, broken" : "cleared, fine")); return *p == 8 ? -1 : 0; } ---8<--- Link: http://lkml.kernel.org/r/20170725101230.5v7gvnjmcnkzzql3@techsingularity.net Signed-off-by: Mel Gorman Reported-by: Nadav Amit Cc: Andy Lutomirski Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memory.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) diff --git a/mm/memory.c b/mm/memory.c index 71c0b6f98a62..1416485e278c 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1513,8 +1513,20 @@ void zap_page_range(struct vm_area_struct *vma, unsigned long start, tlb_gather_mmu(&tlb, mm, start, end); update_hiwater_rss(mm); mmu_notifier_invalidate_range_start(mm, start, end); - for ( ; vma && vma->vm_start < end; vma = vma->vm_next) + for ( ; vma && vma->vm_start < end; vma = vma->vm_next) { unmap_single_vma(&tlb, vma, start, end, NULL); + + /* + * zap_page_range does not specify whether mmap_sem should be + * held for read or write. That allows parallel zap_page_range + * operations to unmap a PTE and defer a flush meaning that + * this call observes pte_none and fails to flush the TLB. + * Rather than adding a complex API, ensure that no stale + * TLB entries exist when this call returns. + */ + flush_tlb_range(vma, start, end); + } + mmu_notifier_invalidate_range_end(mm, start, end); tlb_finish_mmu(&tlb, start, end); }