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[PATCH 4.6 023/203] mnt: Account for MS_RDONLY in fs_fully_visible

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4.6-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric W. Biederman <ebiederm@xmission.com>

commit 695e9df010e40f407f4830dc11d53dce957710ba upstream.

In rare cases it is possible for s_flags & MS_RDONLY to be set but
MNT_READONLY to be clear.  This starting combination can cause
fs_fully_visible to fail to ensure that the new mount is readonly.
Therefore force MNT_LOCK_READONLY in the new mount if MS_RDONLY
is set on the source filesystem of the mount.

In general both MS_RDONLY and MNT_READONLY are set at the same for
mounts so I don't expect any programs to care.  Nor do I expect
MS_RDONLY to be set on proc or sysfs in the initial user namespace,
which further decreases the likelyhood of problems.

Which means this change should only affect system configurations by
paranoid sysadmins who should welcome the additional protection
as it keeps people from wriggling out of their policies.

Fixes: 8c6cf9cc829f ("mnt: Modify fs_fully_visible to deal with locked ro nodev and atime")
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/namespace.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -3245,6 +3245,10 @@ static bool fs_fully_visible(struct file
 		if (mnt->mnt.mnt_sb->s_iflags & SB_I_NOEXEC)
 			mnt_flags &= ~(MNT_LOCK_NOSUID | MNT_LOCK_NOEXEC);
 
+		/* Don't miss readonly hidden in the superblock flags */
+		if (mnt->mnt.mnt_sb->s_flags & MS_RDONLY)
+			mnt_flags |= MNT_LOCK_READONLY;
+
 		/* Verify the mount flags are equal to or more permissive
 		 * than the proposed new mount.
 		 */

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[PATCH 4.6 000/203] 4.6.5-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:30 +0200
  [PATCH 4.6 017/203] powerpc/tm: Always reclaim in start_thread() for exec() class syscalls Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 014/203] powerpc/iommu: Remove the dependency on EEH struct in DDW mechanism Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 023/203] mnt: Account for MS_RDONLY in fs_fully_visible Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 022/203] mnt: fs_fully_visible test the proper mount for MNT_LOCKED Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 010/203] IB/core: Fix bit curruption in ib_device_cap_flags structure Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 018/203] usb: dwc2: fix regression on big-endian PowerPC/ARM systems Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 002/203] mac80211: fix fast_tx header alignment Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 012/203] IB/rdmavt: Correct qp_priv_alloc() return value test Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 019/203] USB: EHCI: declare hostpc register as zero-length array Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 013/203] IB/mlx4: Properly initialize GRH TClass and FlowLabel in AHs Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 016/203] powerpc/pseries: Fix IBM_ARCH_VEC_NRCORES_OFFSET since POWER8NVL was added Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 025/203] of: fix autoloading due to broken modalias with no compatible Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 020/203] USB: dont free bandwidth_mutex too early Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  [PATCH 4.6 015/203] powerpc/pseries: Fix PCI config address for DDW Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 00:40 +0200
  Re: [PATCH 4.6 000/203] 4.6.5-stable review Shuah Khan <shuah.kh@samsung.com> - 2016-07-26 04:10 +0200
    Re: [PATCH 4.6 000/203] 4.6.5-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 04:50 +0200
  Re: [PATCH 4.6 000/203] 4.6.5-stable review Guenter Roeck <linux@roeck-us.net> - 2016-07-26 16:00 +0200
    Re: [PATCH 4.6 000/203] 4.6.5-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-26 16:30 +0200
      Re: [PATCH 4.6 000/203] 4.6.5-stable review Guenter Roeck <linux@roeck-us.net> - 2016-07-26 17:50 +0200
  Re: [PATCH 4.6 000/203] 4.6.5-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-07-27 06:50 +0200

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