RFR: 8308503: AArch64: SIGILL when running with -XX:UseBranchProtection=pac-ret on hardware without PAC feature

[Andrew Haley]

On Tue, 23 May 2023 05:20:58 GMT, Hao Sun <haosun at openjdk.org> wrote:

> When revisiting the behavior of UseBranchProtection [1], we get one SIGILL error when running with -XX:UseBranchProtection=pac-ret on hardware without PAC.
> 
> Problem:
> 
> We build and run `java --version` with the following configuration matrix `Config X VMoption X Machine`.
> 
> 
>   Config = {--enable-branch-protection, null}
>   VMoption = {-XX:UseBranchProtection=pac-ret, -XX:UseBranchProtection=standard}
>   Machine = {w/ PAC, w/o PAC}
> 
> 
> VM crashes with SIGILL error for configure `Config=null, VMoption=pac-ret, Machine=w/o PAC`. The unrecognized instruction is `pacia x30, x29`, i.e. `pacia(lr, rfp)` generated by function `MacroAssembler::protect_return_address()`. [2]
> 
> Root cause:
> 
> 1. Instruction `pacia` is not in the NOP space. That's why `Config=null, VMoption=pac-ret` passes on `hardware w/ PAC`, but fails on `hardware w/o PAC`.
> 
> 2. -XX:UseBranchProtection=pac-ret behaves differently from the document [3], i.e.
> 
> 
>   In order to use Branch Protection features in the VM,
>   --enable-branch-protection must be used
> 
> 
> `_rop_protection` is not turned off for `Config=null`. That's why `VMoption=pac-ret, Machine=w/o PAC` passes with
> `Config=--enable-branch-protection` but fails with `Config=null`.
> 
> Fix:
> 
> This patch refines the parsing of -XX:UseBranchProtection=pac-ret:
> 
> 1. We handle "pac-ret" and "standard" in the same way, since only one type of branch protection is implemented for now, i.e. "pac-ret". We may update "standard" in the future if "bti" protection is added.
> 
> 2. `_rop_protection` is not turned on unless all the three conditions are satisfied [4]. Otherwise, it's kept off and one warning message is emitted.
> 
> 
> // Enable PAC if this code has been built with branch-protection, the
> // CPU/OS supports it, and incompatible preview features aren't enabled.
> 
> 
> [1] https://bugs.openjdk.org/browse/JDK-8287325?focusedCommentId=14581099&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-14581099
> [2] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L5976
> [3] https://github.com/openjdk/jdk/blob/master/doc/building.md#branch-protection
> [4] https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/vm_version_aarch64.cpp#L457

This looks good to me, but please read https://bugs.openjdk.org/browse/JDK-8287325 before you commit anything.

> > This looks good to me, but please read https://bugs.openjdk.org/browse/JDK-8287325 before you commit anything.
> 
> Thanks for reviewing this patch.

> As for your comment, do you mean we should fix these two issues in one patch? Thanks.

No, but you do need to align. I added a suggestion above, for clarity.

src/hotspot/cpu/aarch64/vm_version_aarch64.cpp line 466:

> 464:     } else if (Arguments::enable_preview()) {
> 465:       // Not currently compatible with continuation freeze/thaw.
> 466:       warning("ROP-protection is incompatible with virtual threads preview feature. Disabling ROP-protection.");

Suggestion:

      _rop_protection = false;
      warning("ROP-protection is incompatible with virtual threads preview feature. Disabling ROP-protection.");

-------------

PR Review: https://git.openjdk.org/jdk/pull/14095#pullrequestreview-1439228904
PR Comment: https://git.openjdk.org/jdk/pull/14095#issuecomment-1559635551
PR Review Comment: https://git.openjdk.org/jdk/pull/14095#discussion_r1202508622