keys: Fix UAF in key_put()

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: keys: Fix UAF in key_put() Once a key's reference count has been reduced to 0, the garbage collector thread may destroy it at any time and so key_put() is not allowed to touch the key after that point. The most key_put() is normally allowed to do is to touch key_gc_work as that's a static global variable. However, in an effort to speed up the reclamation of quota, this is now done in key_put() once the key's usage is reduced to 0 - but now the code is looking at the key after the deadline, which is forbidden. Fix this by using a flag to indicate that a key can be gc'd now rather than looking at the key's refcount in the garbage collector.

References

• https://git.kernel.org/stable/c/6afe2ea2daec156bd94ad2c5a6f4f4c48240dcd3
• https://git.kernel.org/stable/c/f6a3cf833188e897c97028cd7b926e3f2cb1a8c0
• https://git.kernel.org/stable/c/75845c6c1a64483e9985302793dbf0dfa5f71e32