unpack: MAKE UNPACKING SAFE AGAIN

We have to use Docker's symlink pkg to make unpacking safe when it comes
to symlinks inside path components (and also scoping in some senses). I
also included some cleaning of hdr.Name just for safety. I will add unit
tests for this.

VOTE STERLING ARCHER 2020

Signed-off-by: Aleksa Sarai <asarai@suse.com>

image/layer/tar_extract.go

@@ -28,6 +28,7 @@ import (
 
 	"github.com/Sirupsen/logrus"
 	"github.com/cyphar/umoci/system"
+	"github.com/cyphar/umoci/third_party/symlink"
 )
 
 // applyMetadata applies the state described in tar.Header to the filesystem at
@@ -92,20 +93,27 @@ func applyMetadata(path string, hdr *tar.Header) error {
 // that the layer state is consistent with the layer state that produced the
 // tar archive being iterated over. This does handle whiteouts, so a tar.Header
 // that represents a whiteout will result in the path being removed.
-//
-// FIXME: This is not currently safe against a variety of attacks (such as
-//        symlink attacks and ../../ attacks).
 func unpackEntry(root string, hdr *tar.Header, r io.Reader) error {
 	logrus.WithFields(logrus.Fields{
 		"root": root,
 		"path": hdr.Name,
 		"type": hdr.Typeflag,
 	}).Debugf("unpacking entry")
 
-	// Get directory and filename.
-	// FIXME: Protect against ../ and symlink attacks.
-	path := filepath.Join(root, hdr.Name)
-	dir, file := filepath.Split(path)
+	// Make hdr.Name safe.
+	hdr.Name = filepath.Clean(filepath.Join("/", hdr.Name))
+
+	// Get directory and filename, but we have to safely get the directory
+	// component of the path. FollowSymlinkInScope will evaluate the path
+	// itself, which we don't want (we're clever enough to handle the actual
+	// path being a symlink).
+	unsafePath := filepath.Join(root, hdr.Name)
+	unsafeDir, file := filepath.Split(unsafePath)
+	dir, err := symlink.FollowSymlinkInScope(unsafeDir, root)
+	if err != nil {
+		return err
+	}
+	path := filepath.Join(dir, file)
 
 	// Before we do anything, get the state of dir. Because we might be adding
 	// or removing files, our parent directory might be modified in the

third_party/symlink/LICENSE.APACHE

@@ -0,0 +1,191 @@
+
+                                 Apache License
+                           Version 2.0, January 2004
+                        http://www.apache.org/licenses/
+
+   TERMS AND CONDITIONS FOR USE, REPRODUCTION, AND DISTRIBUTION
+
+   1. Definitions.
+
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+      direction or management of such entity, whether by contract or
+      otherwise, or (ii) ownership of fifty percent (50%) or more of the
+      outstanding shares, or (iii) beneficial ownership of such entity.
+
+      "You" (or "Your") shall mean an individual or Legal Entity
+      exercising permissions granted by this License.
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+      "Source" form shall mean the preferred form for making modifications,
+      including but not limited to software source code, documentation
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+      form, that is based on (or derived from) the Work and for which the
+      editorial revisions, annotations, elaborations, or other modifications
+      represent, as a whole, an original work of authorship. For the purposes
+      of this License, Derivative Works shall not include works that remain
+      separable from, or merely link (or bind by name) to the interfaces of,
+      the Work and Derivative Works thereof.
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+      "Contribution" shall mean any work of authorship, including
+      the original version of the Work and any modifications or additions
+      to that Work or Derivative Works thereof, that is intentionally
+      submitted to Licensor for inclusion in the Work by the copyright owner
+      or by an individual or Legal Entity authorized to submit on behalf of
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+      to the Licensor or its representatives, including but not limited to
+      communication on electronic mailing lists, source code control systems,
+      and issue tracking systems that are managed by, or on behalf of, the
+      Licensor for the purpose of discussing and improving the Work, but
+      excluding communication that is conspicuously marked or otherwise
+      designated in writing by the copyright owner as "Not a Contribution."
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+      "Contributor" shall mean Licensor and any individual or Legal Entity
+      on behalf of whom a Contribution has been received by Licensor and
+      subsequently incorporated within the Work.
+
+   2. Grant of Copyright License. Subject to the terms and conditions of
+      this License, each Contributor hereby grants to You a perpetual,
+      worldwide, non-exclusive, no-charge, royalty-free, irrevocable
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+      where such license applies only to those patent claims licensable
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+      institute patent litigation against any entity (including a
+      cross-claim or counterclaim in a lawsuit) alleging that the Work
+      or a Contribution incorporated within the Work constitutes direct
+      or contributory patent infringement, then any patent licenses
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+
+   4. Redistribution. You may reproduce and distribute copies of the
+      Work or Derivative Works thereof in any medium, with or without
+      modifications, and in Source or Object form, provided that You
+      meet the following conditions:
+
+      (a) You must give any other recipients of the Work or
+          Derivative Works a copy of this License; and
+
+      (b) You must cause any modified files to carry prominent notices
+          stating that You changed the files; and
+
+      (c) You must retain, in the Source form of any Derivative Works
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+      reproduction, and distribution of the Work otherwise complies with
+      the conditions stated in this License.
+
+   5. Submission of Contributions. Unless You explicitly state otherwise,
+      any Contribution intentionally submitted for inclusion in the Work
+      by You to the Licensor shall be under the terms and conditions of
+      this License, without any additional terms or conditions.
+      Notwithstanding the above, nothing herein shall supersede or modify
+      the terms of any separate license agreement you may have executed
+      with Licensor regarding such Contributions.
+
+   6. Trademarks. This License does not grant permission to use the trade
+      names, trademarks, service marks, or product names of the Licensor,
+      except as required for reasonable and customary use in describing the
+      origin of the Work and reproducing the content of the NOTICE file.
+
+   7. Disclaimer of Warranty. Unless required by applicable law or
+      agreed to in writing, Licensor provides the Work (and each
+      Contributor provides its Contributions) on an "AS IS" BASIS,
+      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
+      implied, including, without limitation, any warranties or conditions
+      of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A
+      PARTICULAR PURPOSE. You are solely responsible for determining the
+      appropriateness of using or redistributing the Work and assume any
+      risks associated with Your exercise of permissions under this License.
+
+   8. Limitation of Liability. In no event and under no legal theory,
+      whether in tort (including negligence), contract, or otherwise,
+      unless required by applicable law (such as deliberate and grossly
+      negligent acts) or agreed to in writing, shall any Contributor be
+      liable to You for damages, including any direct, indirect, special,
+      incidental, or consequential damages of any character arising as a
+      result of this License or out of the use or inability to use the
+      Work (including but not limited to damages for loss of goodwill,
+      work stoppage, computer failure or malfunction, or any and all
+      other commercial damages or losses), even if such Contributor
+      has been advised of the possibility of such damages.
+
+   9. Accepting Warranty or Additional Liability. While redistributing
+      the Work or Derivative Works thereof, You may choose to offer,
+      and charge a fee for, acceptance of support, warranty, indemnity,
+      or other liability obligations and/or rights consistent with this
+      License. However, in accepting such obligations, You may act only
+      on Your own behalf and on Your sole responsibility, not on behalf
+      of any other Contributor, and only if You agree to indemnify,
+      defend, and hold each Contributor harmless for any liability
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+      of your accepting any such warranty or additional liability.
+
+   END OF TERMS AND CONDITIONS
+
+   Copyright 2014-2016 Docker, Inc.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.

third_party/symlink/LICENSE.BSD

@@ -0,0 +1,27 @@
+Copyright (c) 2014-2016 The Docker & Go Authors. All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are
+met:
+
+   * Redistributions of source code must retain the above copyright
+notice, this list of conditions and the following disclaimer.
+   * Redistributions in binary form must reproduce the above
+copyright notice, this list of conditions and the following disclaimer
+in the documentation and/or other materials provided with the
+distribution.
+   * Neither the name of Google Inc. nor the names of its
+contributors may be used to endorse or promote products derived from
+this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+"AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

third_party/symlink/NOTICE

@@ -0,0 +1,17 @@
+runc
+
+Copyright 2012-2015 Docker, Inc.
+
+This product includes software developed at Docker, Inc. (http://www.docker.com).
+
+The following is courtesy of our legal counsel:
+
+
+Use and transfer of Docker may be subject to certain restrictions by the
+United States and other governments.
+It is your responsibility to ensure that your use and/or transfer does not
+violate applicable laws.
+
+For more information, please see http://www.bis.doc.gov
+
+See also http://www.apache.org/dev/crypto.html and/or seek legal counsel.

third_party/symlink/README.md

@@ -0,0 +1,15 @@
+## `third_party/symlink` ##
+
+This came from `github.com/docker/docker/pkg/symlink`, and has some
+modifications to remove windows support and also remove the usage of Docker's
+`system` pkg.
+
+Package symlink implements EvalSymlinksInScope which is an extension of
+filepath.EvalSymlinks, as well as a Windows long-path aware version of
+filepath.EvalSymlinks from the [Go standard library][filepath].
+
+The code from filepath.EvalSymlinks has been adapted in `fs.go`. Please read
+the `LICENSE.BSD` file that governs `fs{,_unix}.go` and `LICENSE.APACHE` for
+`fs_unix_test.go`.
+
+[filepath]: https://golang.org/pkg/path/filepath