CVE-2021-41220: Use after free in `CollectiveReduceV2`

Impact The async implementation of CollectiveReduceV2 suffers from a memory leak and a use after free:

python import tensorflow as tf tf.rawops.CollectiveReduceV2( input=[], groupsize=[-10, -10, -10], groupkey=[-10, -10], instancekey=[-10], orderingtoken=[], mergeop='Mul', finalop='Div')

This occurs due to the asynchronous computation and the fact that objects that have been std::move()d from are still accessed:

cc auto donewithcleanup = colparams, done = std::move(done) { done(); colparams->Unref(); }; OPREQUIRESOKASYNC(c, FillCollectiveParams(colparams, REDUCTIONCOLLECTIVE, /groupsize/ c->input(1), /groupkey/ c->input(2), /instancekey/ c->input(3)), done);

Here, done is already moved from by the time OPREQUIRESOKASYNC macro needs to invoke it in case of errors. In this case, we get an undefined behavior, which can manifest via crashes, std::badalloc throws or just memory leaks.

Patches We have patched the issue in GitHub commit ca38dab9d3ee66c5de06f11af9a4b1200da5ef75.

The fix will be included in TensorFlow 2.7.0. We will also cherrypick this commit on TensorFlow 2.6.1, as this version is the only one that is also affected.

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Attribution This vulnerability has been reported by members of the Aivul Team from Qihoo 360.