Commit 66fc1303 authored Jun 14, 2015 by Hugh Dickins Committed by Linus Torvalds Jun 17, 2015

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 c7277090 ("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-and-tested-by: Daniel Wagner <wagi@monom.org>
Reported-and-tested-by: Morten Stevens <mstevens@fedoraproject.org>
Signed-off-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 0f57d867

mm/shmem.c

+7 −1

Original line number	Diff line number	Diff line
		@@ -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);