ptrace: slightly saner 'get_dumpable()' logic

In the Linux kernel, the following vulnerability has been resolved: ptrace: slightly saner 'get_dumpable()' logic The 'dumpability' of a task is fundamentally about the memory image of the task the concept comes from whether it can core dump or not and makes no sense when you don't have an associated mm. And almost all users do in fact use it only for the case where the task has a mm pointer. But we have one odd special case: ptrace_may_access() uses 'dumpable' to check various other things entirely independently of the MM (typically explicitly using flags like PTRACE_MODE_READ_FSCREDS). Including for threads that no longer have a VM (and maybe never did, like most kernel threads). It's not what this flag was designed for, but it is what it is. The ptrace code does check that the uid/gid matches, so you do have to be uid-0 to see kernel thread details, but this means that the traditional "drop capabilities" model doesn't make any difference for this all. Make it all make a bit more sense by saying that if you don't have a MM pointer, we'll use a cached "last dumpability" flag if the thread ever had a MM (it will be zero for kernel threads since it is never set), and r