On Wed, 10 Nov 2021 12:32:53 GMT, Alan Hayward <d...@openjdk.java.net> wrote:
> PAC is an optional feature in AArch64 8.3 and is compulsory in v9. One
> of its uses is to protect against ROP based attacks. This is done by
> signing the Link Register whenever it is stored on the stack, and
> authenticating the value when it is loaded back from the stack. If an
> attacker were to try to change control flow by editing the stack then
> the authentication check of the Link Register will fail, causing a
> segfault when the function returns.
>
> On a system with PAC enabled, it is expected that all applications will
> be compiled with ROP protection. Fedora 33 and upwards already provide
> this. By compiling for ARMv8.0, GCC and LLVM will only use the set of
> PAC instructions that exist in the NOP space - on hardware without PAC,
> these instructions act as NOPs, allowing backward compatibility for
> negligible performance cost (2 NOPs per non-leaf function).
>
> Hardware is currently limited to the Apple M1 MacBooks. All testing has
> been done within a Fedora Docker image. A run of SpecJVM showed no
> difference to that of noise - which was surprising.
>
> The most important part of this patch is simply compiling using branch
> protection provided by GCC/LLVM. This protects all C++ code from being
> used in ROP attacks, removing all static ROP gadgets from use.
>
> The remainder of the patch adds ROP protection to runtime generated
> code, in both stubs and compiled Java code. Attacks here are much harder
> as ROP gadgets must be found dynamically at runtime. If/when AOT
> compilation is added to JDK, then all stubs and compiled Java will be
> susceptible ROP gadgets being found by static analysis and therefore
> potentially as vulnerable as C++ code.
>
> There are a number of places where the VM changes control flow by
> rewriting the stack or otherwise. I’ve done some analysis as to how
> these could also be used for attacks (which I didn’t want to post here).
> These areas can be protected ensuring the pointers to various stubs and
> entry points are stored in memory as signed pointers. These changes are
> simple to make (they can be reduced to a type change in common code and
> a few addition sign/auth calls in the backend), but there a lot of them
> and the total code change is fairly large. I’m happy to provide a few
> work in progress patches.
>
> In order to match the security benefits of the Apple Arm64e ABI across
> the whole of JDK, then all the changes mentioned above would be
> required.
src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp line 5185:
> 5183: // ROP Protection
> 5184:
> 5185: void MacroAssembler::protect_return_address() {
We need proper, full, detailed comments about what these functions do, with
reference to primary AArch64 documentation.
-------------
PR: https://git.openjdk.java.net/jdk/pull/6334
