Secure usage of Emscripten heap #495
Conversation
Provide some wrapper functions to hide all the intricacies of dealing with Emscrpten heap. We should use these functions instead of calling C directly. Note that they ensure that allocated memory is zeroed out before use as well as after use. This makes it easier to debug initialization issues, and reduces the likelyhood of sensitive data leakage through Emscripten heap.
Instead of calling malloc() and free() directly, use our new helpers to deal with passing byte arrays between JavaScript and Emscripten.
|
Currently this covers only key generation. I'll update other cryptosystems in this PR once they are merged. |
Instead of calling malloc() and free() directly, use our new helpers to deal with passing byte arrays between JavaScript and Emscripten.
Instead of calling malloc() and free() directly, use our new helpers to deal with passing byte arrays between JavaScript and Emscripten.
Instead of calling malloc() and free() directly, use our new helpers to deal with passing byte arrays between JavaScript and Emscripten.
Instead of calling malloc() and free() directly, use our new helpers to deal with passing byte arrays between JavaScript and Emscripten.
|
I've converted all allocations to use |
We already do it in this PR. These calls are placed in finally blocks so they are executed in both the happy path and when an exception is thrown due to an error. |
|
Oh, i missed that we wrap everything in |
heapAlloc: if malloc() fails then do not zero out the result heapFree: make this function a no-op to allow its usage in failure paths where the pointer is null (e.g., returned by failed heapAlloc) We use a weird check "!!buffer" because JavaScript has weird ideas about what is null and what is zero. malloc() results are generally checked with "!ptr" so we invert this check.
|
Your comment made me realize that we handle memory allocation errors poorly in the these utility functions. I've added a minor fix which allows using |
|
that's actually makes sense! 💪 |
In order to call Themis Core functions we need to transfer key material and messages from JavaScript memory into Emscripten heap. Operations results are similarly transferred back to JavaScript memory. We do this by explicitly allocating a buffer on Emscripten heap and using Emscripten helpers for transfers.
One important point here is that all data is still retained for some time on Emscripten heap after
free()is called. This includes sensitive data which theoretically may be accessed by an adversary. Generally it is recommended to wipe the memory that contains sensitive data after use. We can't control JavaScript memory, but we can at least ensure that Emscripten heap does not leave any unwanted traces.Introduce some functions to help us with data transfer between JavaScript and Emscripten heaps. They are designed to be used together so any new code will naturally use them instead of reaching for C functions and Emscripten runtime directly.