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ovl: check whiteout in ovl_create_over_whiteout()

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Kaixuxia repors that it's possible to crash overlayfs by removing the
whiteout on the upper layer before creating a directory over it.  This is a
reproducer:

 mkdir lower upper work merge
 touch lower/file
 mount -t overlay overlay -olowerdir=lower,upperdir=upper,workdir=work merge
 rm merge/file
 ls -al merge/file
 rm upper/file
 ls -al merge/
 mkdir merge/file

Before commencing with a vfs_rename(..., RENAME_EXCHANGE) verify that the
lookup of "upper" is positive and is a whiteout, and return ESTALE
otherwise.

Reported by: kaixuxia <xiakaixu1987@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: e9be9d5e76 ("overlay filesystem")
Cc: <stable@vger.kernel.org> # v3.18
This commit is contained in:
Miklos Szeredi 2018-10-31 12:15:23 +01:00
parent 14fa085640
commit 5e12758086
1 changed files with 4 additions and 0 deletions
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4
fs/overlayfs/dir.c
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@ -462,6 +462,10 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
if (IS_ERR(upper))
goto out_unlock;
err = -ESTALE;
if (d_is_negative(upper) || !IS_WHITEOUT(d_inode(upper)))
goto out_dput;
newdentry = ovl_create_temp(workdir, cattr);
err = PTR_ERR(newdentry);
if (IS_ERR(newdentry))