From df2cc96e77011cf7989208b206da9817e0321028 Mon Sep 17 00:00:00 2001 From: Mike Rapoport Date: Thu, 7 Jun 2018 17:09:25 -0700 Subject: userfaultfd: prevent non-cooperative events vs mcopy_atomic races If a process monitored with userfaultfd changes it's memory mappings or forks() at the same time as uffd monitor fills the process memory with UFFDIO_COPY, the actual creation of page table entries and copying of the data in mcopy_atomic may happen either before of after the memory mapping modifications and there is no way for the uffd monitor to maintain consistent view of the process memory layout. For instance, let's consider fork() running in parallel with userfaultfd_copy(): process | uffd monitor ---------------------------------+------------------------------ fork() | userfaultfd_copy() ... | ... dup_mmap() | down_read(mmap_sem) down_write(mmap_sem) | /* create PTEs, copy data */ dup_uffd() | up_read(mmap_sem) copy_page_range() | up_write(mmap_sem) | dup_uffd_complete() | /* notify monitor */ | If the userfaultfd_copy() takes the mmap_sem first, the new page(s) will be present by the time copy_page_range() is called and they will appear in the child's memory mappings. However, if the fork() is the first to take the mmap_sem, the new pages won't be mapped in the child's address space. If the pages are not present and child tries to access them, the monitor will get page fault notification and everything is fine. However, if the pages *are present*, the child can access them without uffd noticing. And if we copy them into child it'll see the wrong data. Since we are talking about background copy, we'd need to decide whether the pages should be copied or not regardless #PF notifications. Since userfaultfd monitor has no way to determine what was the order, let's disallow userfaultfd_copy in parallel with the non-cooperative events. In such case we return -EAGAIN and the uffd monitor can understand that userfaultfd_copy() clashed with a non-cooperative event and take an appropriate action. Link: http://lkml.kernel.org/r/1527061324-19949-1-git-send-email-rppt@linux.vnet.ibm.com Signed-off-by: Mike Rapoport Acked-by: Pavel Emelyanov Cc: Andrea Arcangeli Cc: Mike Kravetz Cc: Andrei Vagin Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/userfaultfd.c | 22 +++++++++++++++++----- 1 file changed, 17 insertions(+), 5 deletions(-) (limited to 'mm') diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 39791b8..5029f24 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -404,7 +404,8 @@ static __always_inline ssize_t __mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start, unsigned long src_start, unsigned long len, - bool zeropage) + bool zeropage, + bool *mmap_changing) { struct vm_area_struct *dst_vma; ssize_t err; @@ -431,6 +432,15 @@ retry: down_read(&dst_mm->mmap_sem); /* + * If memory mappings are changing because of non-cooperative + * operation (e.g. mremap) running in parallel, bail out and + * request the user to retry later + */ + err = -EAGAIN; + if (mmap_changing && READ_ONCE(*mmap_changing)) + goto out_unlock; + + /* * Make sure the vma is not shared, that the dst range is * both valid and fully within a single existing vma. */ @@ -563,13 +573,15 @@ out: } ssize_t mcopy_atomic(struct mm_struct *dst_mm, unsigned long dst_start, - unsigned long src_start, unsigned long len) + unsigned long src_start, unsigned long len, + bool *mmap_changing) { - return __mcopy_atomic(dst_mm, dst_start, src_start, len, false); + return __mcopy_atomic(dst_mm, dst_start, src_start, len, false, + mmap_changing); } ssize_t mfill_zeropage(struct mm_struct *dst_mm, unsigned long start, - unsigned long len) + unsigned long len, bool *mmap_changing) { - return __mcopy_atomic(dst_mm, start, 0, len, true); + return __mcopy_atomic(dst_mm, start, 0, len, true, mmap_changing); } -- cgit v1.1