security(state): Limit the number of non-finalized chains tracked by Zebra

[teor2345]

Motivation

There is no limit on the number of non-finalized chains that Zebra can track.

Theoretically, this allows unbounded use of memory. But this bug is difficult to exploit on mainnet, because it would require mining hundreds of side-chain blocks at mainnet difficulty.

Specifications

The number of chains tracked by a node implementation is a standard rule, not a consensus rule. For zcashd, it is currently 1 in memory, and some number on disk.

For Zebra, after this PR it will be 10 in memory. (Zebra doesn't store any unverified or side-chain blocks on disk.)

Complex Code or Requirements

This is concurrent code, the non-finalized state is cloned before reading it concurrently. We ensure that the limit of 10 is kept by making the list of chains into a private field, and only accessing them via a method.

Solution

• Limit the number of non-finalized chains to 10
  • If the limit is reached, drop the chain with the least work
  • Make the chain list private, and provide iterator, count, and insert chain methods

Review

This is a low priority security fix.

Reviewer Checklist

• Will the PR name make sense to users?
  • Does it need extra CHANGELOG info? (new features, breaking changes, large changes)
• Are the PR labels correct?
• Does the code do what the ticket and PR says?
  • Does it change concurrent code, unsafe code, or consensus rules?
• How do you know it works? Does it have tests?

[codecov]

Codecov Report

Merging #6447 (de1269f) into main (5db2243) will increase coverage by 0.07%.
The diff coverage is 90.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6447      +/-   ##
==========================================
+ Coverage   77.78%   77.86%   +0.07%     
==========================================
  Files         304      304              
  Lines       39637    39659      +22     
==========================================
+ Hits        30833    30879      +46     
+ Misses       8804     8780      -24     

[arya2]

This looks excellent!

[teor2345]

Temporary network errors:

warning: spurious network error (2 tries remaining): early EOF; class=Net (12); code=Eof (-20)
Warning: warning: spurious network error (1 tries remaining): early EOF; class=Net (12); code=Eof (-20)
error: Unable to update registry crates-io
Error: Unable to update registry crates-io
Caused by:
failed to fetch [https://github.com/rust-lang/crates.io-index](https://github.com/rust-lang/crates.io-index%60)

https://github.com/ZcashFoundation/zebra/actions/runs/4601125572/jobs/8128642182#step:5:15

Error: -03 20:40:13 [ERROR] cargo metadata exited with an error: Updating crates.io index
warning: spurious network error (2 tries remaining): early EOF; class=Net (12); code=Eof (-20)
warning: spurious network error (1 tries remaining): early EOF; class=Net (12); code=Eof (-20)
error: Unable to update registry crates-io

Caused by:
failed to fetch [https://github.com/rust-lang/crates.io-index](https://github.com/rust-lang/crates.io-index%60)

https://github.com/ZcashFoundation/zebra/actions/runs/4601125562/jobs/8128592665?pr=6447#step:4:14