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[PATCH 4.8 11/35] ovl: fix d_real() for stacked fs

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

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

From: Miklos Szeredi <mszeredi@redhat.com>

commit c4fcfc1619ea43a8a89ad2f83ff23905eee088bd upstream.

Handling of recursion in d_real() is completely broken.  Recursion is only
done in the 'inode != NULL' case.  But when opening the file we have
'inode == NULL' hence d_real() will return an overlay dentry.  This won't
work since overlayfs doesn't define its own file operations, so all file
ops will fail.

Fix by doing the recursion first and the check against the inode second.

Bash script to reproduce the issue written by Quentin:

 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -
tmpdir=$(mktemp -d)
pushd ${tmpdir}

mkdir -p {upper,lower,work}
echo -n 'rocks' > lower/ksplice
mount -t overlay level_zero upper -o lowerdir=lower,upperdir=upper,workdir=work
cat upper/ksplice

tmpdir2=$(mktemp -d)
pushd ${tmpdir2}

mkdir -p {upper,work}
mount -t overlay level_one upper -o lowerdir=${tmpdir}/upper,upperdir=upper,workdir=work
ls -l upper/ksplice
cat upper/ksplice
 - 8< - - - - - 8< - - - - - 8< - - - - - 8< - - - -

Reported-by: Quentin Casasnovas <quentin.casasnovas@oracle.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 2d902671ce1c ("vfs: merge .d_select_inode() into .d_real()")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/overlayfs/super.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -329,11 +329,11 @@ static struct dentry *ovl_d_real(struct
 	if (!real)
 		goto bug;
 
+	/* Handle recursion */
+	real = d_real(real, inode, open_flags);
+
 	if (!inode || inode == d_inode(real))
 		return real;
-
-	/* Handle recursion */
-	return d_real(real, inode, open_flags);
 bug:
 	WARN(1, "ovl_d_real(%pd4, %s:%lu): real dentry not found\n", dentry,
 	     inode ? inode->i_sb->s_id : "NULL", inode ? inode->i_ino : 0);

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[PATCH 4.8 00/35] 4.8.13-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 16/35] PCI: Export pcie_find_root_port Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 11/35] ovl: fix d_real() for stacked fs Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 01/35] libata-scsi: Fixup ata_gen_passthru_sense() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 25/35] drm/radeon: fix check for port PM availability Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 14/35] rcu: Fix soft lockup for rcu_nocb_kthread Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 21/35] drm/i915: drop the struct_mutex when wedged or trying to reset Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 02/35] scsi: hpsa: use bus 3 for legacy HBA devices Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 23/35] drm/radeon: fix power state when port pm is unavailable (v2) Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 26/35] arm64: dts: juno: fix cluster sleep state entry latency on all SoC versions Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:10 +0100
  [PATCH 4.8 07/35] thp: fix corner case of munlock() of PTE-mapped THPs Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 10/35] mm: fix false-positive WARN_ON() in truncate/invalidate for hugetlb Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 29/35] drm/mediatek: fix null pointer dereference Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 08/35] zram: fix unbalanced idr management at hot removal Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 35/35] arm64: suspend: Reconfigure PSTATE after resume from idle Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 34/35] arm64: mm: Set PSTATE.PAN from the cpu_enable_pan() call Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 04/35] ARC: mm: PAE40: Fix crash at munmap Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 24/35] drm/amdgpu: fix check for port PM availability Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 20/35] drm/i915: Dont touch NULL sg on i915_gem_object_get_pages_gtt() error Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 05/35] ARC: Dont use "+l" inline asm constraint Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 12/35] Input: change KEY_DATA from 0x275 to 0x277 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 30/35] perf/x86: Restore TASK_SIZE check on frame pointer Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 28/35] pwm: Fix device reference leak Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 03/35] scsi: libfc: fix seconds_since_last_reset miscalculation Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 32/35] batman-adv: Detect missing primaryif during tp_send as error Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 33/35] arm64: cpufeature: Schedule enable() calls instead of calling them via IPI Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  [PATCH 4.8 09/35] kasan: update kasan_global for gcc 7 Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-07 08:20 +0100
  Re: [PATCH 4.8 00/35] 4.8.13-stable review Guenter Roeck <linux@roeck-us.net> - 2016-12-07 17:10 +0100
    Re: [PATCH 4.8 00/35] 4.8.13-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-08 17:30 +0100
  Re: [PATCH 4.8 00/35] 4.8.13-stable review Shuah Khan <shuah.kh@samsung.com> - 2016-12-07 19:20 +0100
    Re: [PATCH 4.8 00/35] 4.8.13-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-08 17:30 +0100
  Re: [PATCH 4.8 00/35] 4.8.13-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2016-12-08 17:30 +0100

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