48 lines
1.9 KiB
Diff
48 lines
1.9 KiB
Diff
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From: Hugh Dickins <hughd@google.com>
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Date: Sun, 14 Jun 2015 09:48:09 -0700
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Subject: [PATCH] mm: shmem_zero_setup skip security check and lockdep conflict
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with XFS
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It appears that, at some point last year, XFS made directory handling
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changes which bring it into lockdep conflict with shmem_zero_setup():
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it is surprising that mmap() can clone an inode while holding mmap_sem,
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but that has been so for many years.
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Since those few lockdep traces that I've seen all implicated selinux,
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I'm hoping that we can use the __shmem_file_setup(,,,S_PRIVATE) which
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v3.13's commit c7277090927a ("security: shmem: implement kernel private
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shmem inodes") introduced to avoid LSM checks on kernel-internal inodes:
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the mmap("/dev/zero") cloned inode is indeed a kernel-internal detail.
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This also covers the !CONFIG_SHMEM use of ramfs to support /dev/zero
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(and MAP_SHARED|MAP_ANONYMOUS). I thought there were also drivers
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which cloned inode in mmap(), but if so, I cannot locate them now.
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Reported-and-tested-by: Prarit Bhargava <prarit@redhat.com>
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Reported-by: Daniel Wagner <wagi@monom.org>
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Reported-by: Morten Stevens <mstevens@fedoraproject.org>
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Signed-off-by: Hugh Dickins <hughd@google.com>
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---
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mm/shmem.c | 8 +++++++-
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1 file changed, 7 insertions(+), 1 deletion(-)
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diff --git a/mm/shmem.c b/mm/shmem.c
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index de981370fbc5..47d536e59fc0 100644
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--- a/mm/shmem.c
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+++ b/mm/shmem.c
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@@ -3401,7 +3401,13 @@ int shmem_zero_setup(struct vm_area_struct *vma)
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struct file *file;
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loff_t size = vma->vm_end - vma->vm_start;
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- file = shmem_file_setup("dev/zero", size, vma->vm_flags);
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+ /*
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+ * Cloning a new file under mmap_sem leads to a lock ordering conflict
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+ * between XFS directory reading and selinux: since this file is only
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+ * accessible to the user through its mapping, use S_PRIVATE flag to
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+ * bypass file security, in the same way as shmem_kernel_file_setup().
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+ */
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+ file = __shmem_file_setup("dev/zero", size, vma->vm_flags, S_PRIVATE);
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if (IS_ERR(file))
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return PTR_ERR(file);
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