fix: block adversarial exploit of non-aborting assert!
#8559
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leodemoura
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May 31, 2025
This PR is similar to #8559, but for `Expr.mkData`
digama0
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May 31, 2025
| let r : UInt64 := h.toUInt32.toUInt64 + hasMVar.toUInt64.shiftLeft 32 + hasParam.toUInt64.shiftLeft 33 + depth.toUInt64.shiftLeft 40 | ||
| r | ||
| @[extern "lean_level_mk_data"] | ||
| opaque Level.mkData (h : UInt64) (depth : Nat := 0) (hasMVar hasParam : Bool := false) : Level.Data |
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Note that it is still possible to prove that Level.Data is finite here, because it is not opaque type, and moreover Level.Data.depth returns a UInt32. If you are going to use this approach, please at least take the modification to lower the max bvar range to 2^20-2, so that it is at least possible to prove that there exists a sound implementation of the function which is indistinguishable from the real one without hitting a panic.
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Or alternatively, make Level.depth and Expr.looseBVarRange opaque, so that it cannot be seen that they are implemented by projecting out of a finite type.
This was referenced Jun 16, 2025
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This PR fixes an adversarial soundness attack described in #8554. The attack exploits the fact that
assert!no longer aborts execution, and that users can redirect error messages.Another PR will implement the same fix for
Expr.Data.