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authorHugh Dickins <hughd@google.com>2015-06-14 09:48:09 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2015-06-17 20:40:19 -1000
commit66fc13039422ba7df2d01a8ee0873e4ef965b50b (patch)
tree0826a86f9b90bf179b462e9d01b7887400a48ee0 /mm
parent0f57d86787d8b1076ea8f9cbdddda2a46d534a27 (diff)
downloadlwn-66fc13039422ba7df2d01a8ee0873e4ef965b50b.tar.gz
lwn-66fc13039422ba7df2d01a8ee0873e4ef965b50b.zip
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@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>
Diffstat (limited to 'mm')
-rw-r--r--mm/shmem.c8
1 files changed, 7 insertions, 1 deletions
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);