4.19-stable review patch. If anyone has any objections, please let me know.
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From: Andrea Arcangeli aarcange@redhat.com
commit dcf7fe9d89763a28e0f43975b422ff141fe79e43 upstream.
Set the page dirty if VM_WRITE is not set because in such case the pte won't be marked dirty and the page would be reclaimed without writepage (i.e. swapout in the shmem case).
This was found by source review. Most apps (certainly including QEMU) only use UFFDIO_COPY on PROT_READ|PROT_WRITE mappings or the app can't modify the memory in the first place. This is for correctness and it could help the non cooperative use case to avoid unexpected data loss.
Link: http://lkml.kernel.org/r/20181126173452.26955-6-aarcange@redhat.com Reviewed-by: Hugh Dickins hughd@google.com Cc: stable@vger.kernel.org Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support") Reported-by: Hugh Dickins hughd@google.com Signed-off-by: Andrea Arcangeli aarcange@redhat.com Cc: "Dr. David Alan Gilbert" dgilbert@redhat.com Cc: Jann Horn jannh@google.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Mike Rapoport rppt@linux.ibm.com Cc: Peter Xu peterx@redhat.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/shmem.c | 11 +++++++++++ 1 file changed, 11 insertions(+)
--- a/mm/shmem.c +++ b/mm/shmem.c @@ -2325,6 +2325,16 @@ static int shmem_mfill_atomic_pte(struct _dst_pte = mk_pte(page, dst_vma->vm_page_prot); if (dst_vma->vm_flags & VM_WRITE) _dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte)); + else { + /* + * We don't set the pte dirty if the vma has no + * VM_WRITE permission, so mark the page dirty or it + * could be freed from under us. We could do it + * unconditionally before unlock_page(), but doing it + * only if VM_WRITE is not set is faster. + */ + set_page_dirty(page); + }
dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
@@ -2358,6 +2368,7 @@ out: return ret; out_release_uncharge_unlock: pte_unmap_unlock(dst_pte, ptl); + ClearPageDirty(page); delete_from_page_cache(page); out_release_uncharge: mem_cgroup_cancel_charge(page, memcg, false);