* Validate faulting addresses are valid to fault on
This commit adds a defense-in-depth measure to Wasmtime which is
intended to mitigate the impact of CVEs such as GHSA-ff4p-7xrq-q5r8.
Currently Wasmtime will catch `SIGSEGV` signals for WebAssembly code so
long as the instruction which faulted is an allow-listed instruction
(aka has a trap code listed for it). With the recent security issue,
however, the problem was that a wasm guest could exploit a compiler bug
to access memory outside of its sandbox. If the access was successful
there's no real way to detect that, but if the access was unsuccessful
then Wasmtime would happily swallow the `SIGSEGV` and report a nominal
trap. To embedders, this might look like nothing is going awry.
The new strategy implemented here in this commit is to attempt to be
more robust towards these sorts of failures. When a `SIGSEGV` is raised
the faulting pc is recorded but additionally the address of the
inaccessible location is also record. After the WebAssembly stack is
unwound and control returns to Wasmtime which has access to a `Store`
Wasmtime will now use this inaccessible faulting address to translate it
to a wasm address. This process should be guaranteed to succeed as
WebAssembly should only be able to access a well-defined region of
memory for all linear memories in a `Store`.
If no linear memory in a `Store` could contain the faulting address,
then Wasmtime now prints a scary message and aborts the process. The
purpose of this is to catch these sorts of bugs, make them very loud
errors, and hopefully mitigate impact. This would continue to not
mitigate the impact of a guest successfully loading data outside of its
sandbox, but if a guest was doing a sort of probing strategy trying to
find valid addresses then any invalid access would turn into a process
crash which would immediately be noticed by embedders.
While I was here I went ahead and additionally took a stab at #3120.
Traps due to `SIGSEGV` will now report the size of linear memory and the
address that was being accessed in addition to the bland "access out of
bounds" error. While this is still somewhat bland in the context of a
high level source language it's hopefully at least a little bit more
actionable for some. I'll note though that this isn't a guaranteed
contextual message since only the default configuration for Wasmtime
generates `SIGSEGV` on out-of-bounds memory accesses. Dynamically
bounds-checked configurations, for example, don't do this.
Testing-wise I unfortunately am not aware of a great way to test this.
The closet equivalent would be something like an `unsafe` method
`Config::allow_wasm_sandbox_escape`. In lieu of adding tests, though, I
can confirm that during development the crashing messages works just
fine as it took awhile on macOS to figure out where the faulting address
was recorded in the exception information which meant I had lots of
instances of recording an address of a trap not accessible from wasm.
* Fix tests
* Review comments
* Fix compile after refactor
* Fix compile on macOS
* Fix trap test for s390x
s390x rounds faulting addresses to 4k boundaries.
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