Fix buffer over-read in FileAccessMemory::get_buffer

[nikitalita]

The bounds check for FileAccessMemory::get_buffer is length - pos, but it is saved to a unsigned integer. This means that if pos is greater than length (which the user can very easily do by seeking past the end), left will underflow and the bounds check will fail, causing a buffer over-read.

Casting the result to an int64_t and clamping it at >= 0 fixes the issue.

[Ivorforce]

If the user can use seek to seek past the end, shouldn't we insert an error check there instead? Seeking past the end seems like erroneous behavior to me.

[nikitalita]

If the user can use seek to seek past the end, shouldn't we insert an error check there instead? Seeking past the end seems like erroneous behavior to me.

It seemed like that to me too, but I didn’t write it and I don’t know what the logic is behind that decision, so I left it. Do you want me to add that?

[Ivorforce]

I'd prefer that as a solution. FileAccessCompressed seems to behave this way too:

godot/core/io/file_access_compressed.cpp

Line 196 in 7864ac8

ERR_FAIL_COND(p_position > read_total);