linux-kernel - mm: shmem_zero_setup skip security check and lockdep conflict with XFS

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Message-ID: <alpine.LSU.2.11.1506140944380.11018@eggly.anvils>
Date:	Sun, 14 Jun 2015 09:48:09 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Prarit Bhargava <prarit@...hat.com>,
	Morten Stevens <mstevens@...oraproject.org>,
	Daniel Wagner <wagi@...om.org>,
	Dave Chinner <david@...morbit.com>,
	Eric Paris <eparis@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: 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 <prarit@...hat.com>
Reported-by: Daniel Wagner <wagi@...om.org>
Reported-by: Morten Stevens <mstevens@...oraproject.org>
Signed-off-by: Hugh Dickins <hughd@...gle.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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