hold reference to SslContextHandle to prevent crashes

[wfurt]

This is speculative fix for #73621 and #69125.
I was unable to reproduce it locally so this is based on code inspection, experiments and discussions on #73621.
The assumption is that this is regression caused by #64369

The theory is like this:
OpenSSL has its own referencing mechanism. The particular SSL structure it can still hold reference to SSL_CTX it was created with. That for example allows to use TLS session cache attached to SSL_CTX.
Now, from .NET prospective the SafeSslContextHandle representing SSL_CTX can be released as soon as we create SafeSslHandle - we don't use it for anything else but initial setup. Calling ReleaseHandle would free the GCHandle and if any of the callbacks fire after that, they can step on memory that holds garbage.

This change grabs reference on SafeSslContextHandle (if eligible for TLS resume) to preserve it while the session is in use to mimic OpenSSL life cycle. With that, the native callbacks should point to valid memory.

The crashes are happening daily in CI so we should know in next few days if this fixes the crashes.

[ghost]

Tagging subscribers to this area: @dotnet/ncl, @vcsjones
See info in area-owners.md if you want to be subscribed.

Issue Details

---

This is speculative fix for #73621 and #69125.
I was unable to reproduce it locally so this is based on code inspection, experiments and discussions on #73621.
The assumption is that this is regression caused by #64369

The theory is like this:
OpenSSL has its own referencing mechanism. The particular SSL structure it can still hold reference to SSL_CTX it was created with. That for example allows to use TLS session cache attached to SSL_CTX.
Now, from .NET prospective the SafeSslContextHandle representing SSL_CTX can be released as soon as we create SafeSslHandle - we don't use it for anything else but initial setup. Calling ReleaseHandle would free the GCHandle and if any of the callbacks fire after that, they can step on memory that holds garbage.

This change grabs reference on SafeSslContextHandle (if eligible for TLS resume) to preserve it while the session is in use to mimic OpenSSL life cycle. With that, the native callbacks should point to valid memory.

The crashes are happening daily in CI so we should know in next few days if this fixes the crashes.

Author:	wfurt
Assignees:	wfurt
Labels:	area-System.Net.Security, os-linux
Milestone:	7.0.0

[karelz]

@wfurt I fixed the milestone -- the PR was merged after 7.0 fork, so it won't be part of 7.0.
If we need it in 7.0, we need to backport it.

[wfurt]

/backport to release/7.0-rc1

[wfurt]

/backport to release/7.0-rc1

[github-actions]

Started backporting to release/7.0-rc1: https://github.com/dotnet/runtime/actions/runs/2906074446