Memory Safety Issue when using patch or merge on state and assign the result back to state

tremor-script (crates.io)
  • >=0.11.6
  • <=0.7.2


Affected versions of this crate maintains references to memory that might have been freed already. If affects the following two tremor-script language constructs:

let state = merge state of event end;
let state = patch state of insert event.key => event.value end;

For constructs like this (it doesnt matter what is references in the expression to be merged or the patch operations) an optimization was applied to manipulate the target value in-place, instead of cloning it.

Our Value struct which underpins all event data in tremor-script, is representing as borrowed strings beef::Cow<'lifetime, str> that reference the actual Vec<u8> the event is based upon. We keep the raw byte-array next to the event structure inside our Event struct as a self-referential struct, so we make sure that the structured Value and its references are valid across its whole lifetime.

The optimization was considered safe as long as it was only possible to merge or patche event data or static data. When state was introduced to tremor-script a new possibility existed, to keep Value data around for longer than the lifetime of an event. If event data is merged or patched into state without cloning state first, it can still reference keys or values from the previous event, which will now be invalid. This allows access to those already freed regions of memory and to get their content out over the wire.


If an upgrade is not possible, a possible workaround is to avoid the optimization by introducing a temporary variable and not immediately reassigning to state:

let tmp = merge state of event end;
let state = tmp


The flaw was corrected in tremor-script version 0.11.6 via commit 1a2efcd by removing the optimization and always clone the target expression of a Merge or [Patch](https://www.tremor.rs/docs/tremor-script/index#patch.