CVE-2021-47131: net/tls: Fix use-after-free after the TLS device goes down and up

In the Linux kernel, the following vulnerability has been resolved:

net/tls: Fix use-after-free after the TLS device goes down and up

When a netdev with active TLS offload goes down, tlsdevicedown is called to stop the offload and tear down the TLS context. However, the socket stays alive, and it still points to the TLS context, which is now deallocated. If a netdev goes up, while the connection is still active, and the data flow resumes after a number of TCP retransmissions, it will lead to a use-after-free of the TLS context.

This commit addresses this bug by keeping the context alive until its normal destruction, and implements the necessary fallbacks, so that the connection can resume in software (non-offloaded) kTLS mode.

On the TX side tlsswfallback is used to encrypt all packets. The RX side already has all the necessary fallbacks, because receiving non-decrypted packets is supported. The thing needed on the RX side is to block resync requests, which are normally produced after receiving non-decrypted packets.

The necessary synchronization is implemented for a graceful teardown: first the fallbacks are deployed, then the driver resources are released (it used to be possible to have a tlsdevresync after tlsdevdel).

A new flag called TLSRXDEVDEGRADED is added to indicate the fallback mode. It's used to skip the RX resync logic completely, as it becomes useless, and some objects may be released (for example, resyncasync, which is allocated and freed by the driver).