Segfault on strings tensors with mistmatched dimensions, due to Go code

### Impact Under certain conditions, Go code can trigger a segfault in string deallocation. For string tensors, `C.TF_TString_Dealloc` is called during garbage collection within a finalizer function. However, tensor structure isn't checked until encoding to avoid a performance penalty. The current method for dealloc assumes that encoding succeeded, but segfaults when a string tensor is garbage collected whose encoding failed (e.g., due to mismatched dimensions). To fix this, the call to set the finalizer function is deferred until `NewTensor` returns and, if encoding failed for a string tensor, deallocs are determined based on bytes written. ### Patches We have patched the issue in GitHub commit [8721ba96e5760c229217b594f6d2ba332beedf22](https://github.com/tensorflow/tensorflow/commit/8721ba96e5760c229217b594f6d2ba332beedf22) (merging [#50508](https://github.com/tensorflow/tensorflow/pull/50508)). The fix will be included in TensorFlow 2.6.0. We will also cherrypick this commit on TensorFlow 2.5.1, which is the other affected version. ### For more information Please consult [our security guide](https://github.com/tensorflow/tensorflow/blob/master/SECURITY.md) for more information regarding the security model and how to contact us with issues and questions. ### Attribution This vulnerability has been reported externally via a [fixing PR](https://github.com/tensorflow/tensorflow/pull/50508).

CVSS 3.1

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Severity from

GitHub (reviewed advisory)

Weakness

CWE-20

Also known as

BIT-tensorflow-2021-37692, CVE-2021-37692, PYSEC-2021-314, PYSEC-2021-605, PYSEC-2021-803

• github.com/tensorflow/tensorflow/security/advisories/GHSA-cmgw-8vpc-rc59
• nvd.nist.gov/vuln/detail/CVE-2021-37692
• github.com/tensorflow/tensorflow/pull/50508
• github.com/tensorflow/tensorflow/commit/8721ba96e5760c229217b594f6d2ba332beedf22
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-cpu/PYSEC-2021-605.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow-gpu/PYSEC-2021-803.yaml
• github.com/pypa/advisory-database/tree/main/vulns/tensorflow/PYSEC-2021-314.yaml
• github.com/tensorflow/tensorflow