From: Hugh Dickins Date: Sun, 14 Jun 2015 09:48:09 -0700 Subject: [PATCH] mm: 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 Reported-by: Daniel Wagner Reported-by: Morten Stevens Signed-off-by: Hugh Dickins --- mm/shmem.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/mm/shmem.c b/mm/shmem.c index de981370fbc5..47d536e59fc0 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -3401,7 +3401,13 @@ int shmem_zero_setup(struct vm_area_struct *vma) 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);