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    SubjectRe: mm: shmem_zero_setup skip security check and lockdep conflict with XFS
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    On Sun, Jun 14, 2015 at 12:48 PM, Hugh Dickins <hughd@google.com> wrote:
    > It appears that, at some point last year, XFS made directory handling
    > changes which bring it into lockdep conflict with shmem_zero_setup():
    > it is surprising that mmap() can clone an inode while holding mmap_sem,
    > but that has been so for many years.
    >
    > Since those few lockdep traces that I've seen all implicated selinux,
    > I'm hoping that we can use the __shmem_file_setup(,,,S_PRIVATE) which
    > v3.13's commit c7277090927a ("security: shmem: implement kernel private
    > shmem inodes") introduced to avoid LSM checks on kernel-internal inodes:
    > the mmap("/dev/zero") cloned inode is indeed a kernel-internal detail.
    >
    > This also covers the !CONFIG_SHMEM use of ramfs to support /dev/zero
    > (and MAP_SHARED|MAP_ANONYMOUS). I thought there were also drivers
    > which cloned inode in mmap(), but if so, I cannot locate them now.

    This causes a regression for SELinux (please, in the future, cc
    selinux list and Paul Moore on SELinux-related changes). In
    particular, this change disables SELinux checking of mprotect
    PROT_EXEC on shared anonymous mappings, so we lose the ability to
    control executable mappings. That said, we are only getting that
    check today as a side effect of our file execute check on the tmpfs
    inode, whereas it would be better (and more consistent with the
    mmap-time checks) to apply an execmem check in that case, in which
    case we wouldn't care about the inode-based check. However, I am
    unclear on how to correctly detect that situation from
    selinux_file_mprotect() -> file_map_prot_check(), because we do have a
    non-NULL vma->vm_file so we treat it as a file execute check. In
    contrast, if directly creating an anonymous shared mapping with
    PROT_EXEC via mmap(...PROT_EXEC...), selinux_mmap_file is called with
    a NULL file and therefore we end up applying an execmem check.

    >
    > Reported-and-tested-by: Prarit Bhargava <prarit@redhat.com>
    > Reported-by: Daniel Wagner <wagi@monom.org>
    > Reported-by: Morten Stevens <mstevens@fedoraproject.org>
    > Signed-off-by: Hugh Dickins <hughd@google.com>
    > ---
    >
    > mm/shmem.c | 8 +++++++-
    > 1 file changed, 7 insertions(+), 1 deletion(-)
    >
    > --- 4.1-rc7/mm/shmem.c 2015-04-26 19:16:31.352191298 -0700
    > +++ linux/mm/shmem.c 2015-06-14 09:26:49.461120166 -0700
    > @@ -3401,7 +3401,13 @@ int shmem_zero_setup(struct vm_area_stru
    > struct file *file;
    > loff_t size = vma->vm_end - vma->vm_start;
    >
    > - file = shmem_file_setup("dev/zero", size, vma->vm_flags);
    > + /*
    > + * Cloning a new file under mmap_sem leads to a lock ordering conflict
    > + * between XFS directory reading and selinux: since this file is only
    > + * accessible to the user through its mapping, use S_PRIVATE flag to
    > + * bypass file security, in the same way as shmem_kernel_file_setup().
    > + */
    > + file = __shmem_file_setup("dev/zero", size, vma->vm_flags, S_PRIVATE);
    > if (IS_ERR(file))
    > return PTR_ERR(file);
    >
    > --
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