ovl: check whiteout in ovl_create_over_whiteout()
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
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@ -462,6 +462,10 @@ static int ovl_create_over_whiteout(struct dentry *dentry, struct inode *inode,
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if (IS_ERR(upper))
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goto out_unlock;
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err = -ESTALE;
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if (d_is_negative(upper) || !IS_WHITEOUT(d_inode(upper)))
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goto out_dput;
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newdentry = ovl_create_temp(workdir, cattr);
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err = PTR_ERR(newdentry);
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if (IS_ERR(newdentry))
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