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Andrew Cooper
01e7477d1b
Refer to Intel SDM Rev 70 (Dec 2022), Vol3 17.2.3 "Supervisor Shadow Stack Token". Architecturally, an event delivery which starts in CPL<3 and switches shadow stack will first validate the Supervisor Shadow Stack Token (setting the busy bit), then pushes CS/LIP/SSP. One example of this is an NMI interrupting Xen. Some CPUs suffer from an issue called fracturing, whereby a fault/vmexit/etc between setting the busy bit and completing the event injection renders the action non-restartable, because when it comes time to restart, the busy bit is found to be already set. This is far more easily encountered under virt, yet it is not the fault of the hypervisor, nor the fault of the guest kernel. The fault lies somewhere between the architectural specification, and the uarch behaviour. Intel have allocated CPUID.7[1].ecx[18] CET_SSS to enumerate that supervisor shadow stacks are safe to use. Because of how Xen lays out its shadow stacks, fracturing is not expected to be a problem on native. Detect this case on boot and default to not using shstk if virtualised. Specifying `cet=shstk` on the command line will override this heuristic and enable shadow stacks irrespective. Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com> Reviewed-by: Jan Beulich <jbeulich@suse.com>
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