Signals are a horrid little mechanism.  They are especially nasty in
multi-threaded environments because signal state like handlers is global
across the entire process.  But, signals are basically the only way that
userspace can “gracefully” handle and recover from exceptions.

The kernel generally does not like exceptions to occur during execution.
But, exceptions are a fact of life and must be handled in some
circumstances.  The kernel handles them by keeping a list of individual
instructions which may cause exceptions.  Instead of truly handling the
exception and returning to the instruction that caused it, the kernel
instead restarts execution at a *different* instruction.  This makes it
obvious to that thread of execution that the exception occurred and lets
*that* code handle the exception instead of the handler.

This is not dissimilar to the try/catch exceptions mechanisms that some
programming languages have, but applied *very* surgically to single
instructions.  It effectively changes the visible architecture of the
instruction.

Problem
=======

SGX generates a lot of signals, and the code to enter and exit enclaves and
muck with signal handling is truly horrid.  At the same time, an approach
like kernel exception fixup can not be easily applied to userspace
instructions because it changes the visible instruction architecture.

Solution
========

The vDSO is a special page of kernel-provided instructions that run in
userspace.  Any userspace calling into the vDSO knows that it is special.
This allows the kernel a place to legitimately rewrite the user/kernel
contract and change instruction behavior.

Add support for fixing up exceptions that occur while executing in the
vDSO.  This replaces what could traditionally only be done with signal
handling.

This new mechanism will be used to replace previously direct use of SGX
instructions by userspace.

Just introduce the vDSO infrastructure.  Later patches will actually
replace signal generation with vDSO exception fixup.
Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Jethro Beekman <jethro@fortanix.com>
Link: https://lkml.kernel.org/r/20201112220135.165028-17-jarkko@kernel.org