From: Andrea Arcangeli aarcange@redhat.com Subject: userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas

After the VMA to register the uffd onto is found, check that it has VM_MAYWRITE set before allowing registration. This way we inherit all common code checks before allowing to fill file holes in shmem and hugetlbfs with UFFDIO_COPY.

The userfaultfd memory model is not applicable for readonly files unless it's a MAP_PRIVATE.

Link: http://lkml.kernel.org/r/20181126173452.26955-4-aarcange@redhat.com Fixes: ff62a3421044 ("hugetlb: implement memfd sealing") Signed-off-by: Andrea Arcangeli aarcange@redhat.com Reviewed-by: Mike Rapoport rppt@linux.ibm.com Reviewed-by: Hugh Dickins hughd@google.com Reported-by: Jann Horn jannh@google.com Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support") Cc: stable@vger.kernel.org Cc: "Dr. David Alan Gilbert" dgilbert@redhat.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: Peter Xu peterx@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

fs/userfaultfd.c | 15 +++++++++++++++ mm/userfaultfd.c | 15 ++++++--------- 2 files changed, 21 insertions(+), 9 deletions(-)

--- a/fs/userfaultfd.c~userfaultfd-shmem-hugetlbfs-only-allow-to-register-vm_maywrite-vmas +++ a/fs/userfaultfd.c @@ -1361,6 +1361,19 @@ static int userfaultfd_register(struct u ret = -EINVAL; if (!vma_can_userfault(cur)) goto out_unlock; + + /* + * UFFDIO_COPY will fill file holes even without + * PROT_WRITE. This check enforces that if this is a + * MAP_SHARED, the process has write permission to the backing + * file. If VM_MAYWRITE is set it also enforces that on a + * MAP_SHARED vma: there is no F_WRITE_SEAL and no further + * F_WRITE_SEAL can be taken until the vma is destroyed. + */ + ret = -EPERM; + if (unlikely(!(cur->vm_flags & VM_MAYWRITE))) + goto out_unlock; + /* * If this vma contains ending address, and huge pages * check alignment. @@ -1406,6 +1419,7 @@ static int userfaultfd_register(struct u BUG_ON(!vma_can_userfault(vma)); BUG_ON(vma->vm_userfaultfd_ctx.ctx && vma->vm_userfaultfd_ctx.ctx != ctx); + WARN_ON(!(vma->vm_flags & VM_MAYWRITE));

/* * Nothing to do: this vma is already registered into this @@ -1552,6 +1566,7 @@ static int userfaultfd_unregister(struct cond_resched();

BUG_ON(!vma_can_userfault(vma)); + WARN_ON(!(vma->vm_flags & VM_MAYWRITE));

/* * Nothing to do: this vma is already registered into this --- a/mm/userfaultfd.c~userfaultfd-shmem-hugetlbfs-only-allow-to-register-vm_maywrite-vmas +++ a/mm/userfaultfd.c @@ -205,8 +205,9 @@ retry: if (!dst_vma || !is_vm_hugetlb_page(dst_vma)) goto out_unlock; /* - * Only allow __mcopy_atomic_hugetlb on userfaultfd - * registered ranges. + * Check the vma is registered in uffd, this is + * required to enforce the VM_MAYWRITE check done at + * uffd registration time. */ if (!dst_vma->vm_userfaultfd_ctx.ctx) goto out_unlock; @@ -459,13 +460,9 @@ retry: if (!dst_vma) goto out_unlock; /* - * Be strict and only allow __mcopy_atomic on userfaultfd - * registered ranges to prevent userland errors going - * unnoticed. As far as the VM consistency is concerned, it - * would be perfectly safe to remove this check, but there's - * no useful usage for __mcopy_atomic ouside of userfaultfd - * registered ranges. This is after all why these are ioctls - * belonging to the userfaultfd and not syscalls. + * Check the vma is registered in uffd, this is required to + * enforce the VM_MAYWRITE check done at uffd registration + * time. */ if (!dst_vma->vm_userfaultfd_ctx.ctx) goto out_unlock; _