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Add a KVM-defined ONE_REG register, KVM_REG_GUEST_SSP, to let userspace save and restore the guest's Shadow Stack Pointer (SSP). On both Intel and AMD, SSP is a hardware register that can only be accessed by software via dedicated ISA (e.g. RDSSP) or via VMCS/VMCB fields (used by hardware to context switch SSP at entry/exit). As a result, SSP doesn't fit in any of KVM's existing interfaces for saving/restoring state. Internally, treat SSP as a fake/synthetic MSR, as the semantics of writes to SSP follow that of several other Shadow Stack MSRs, e.g. the PLx_SSP MSRs. Use a translation layer to hide the KVM-internal MSR index so that the arbitrary index doesn't become ABI, e.g. so that KVM can rework its implementation as needed, so long as the ONE_REG ABI is maintained. Explicitly reject accesses to SSP if the vCPU doesn't have Shadow Stack support to avoid running afoul of ignore_msrs, which unfortunately applies to host-initiated accesses (which is a discussion for another day). I.e. ensure consistent behavior for KVM-defined registers irrespective of ignore_msrs. Link: https://lore.kernel.org/all/aca9d389-f11e-4811-90cf-d98e345a5cc2@intel.com Suggested-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com> Tested-by: Mathias Krause <minipli@grsecurity.net> Tested-by: John Allen <john.allen@amd.com> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com> Signed-off-by: Chao Gao <chao.gao@intel.com> Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com> Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com> Link: https://lore.kernel.org/r/20250919223258.1604852-14-seanjc@google.com Co-developed-by: Sean Christopherson <seanjc@google.com> Signed-off-by: Sean Christopherson <seanjc@google.com>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.