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Subjectmm: shmem_zero_setup skip security check and lockdep conflict with XFS
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.

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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