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commitc1f7fec1ebupstream. The bits set in x86_spec_ctrl_mask are used to calculate the guest's value of SPEC_CTRL that is written to the MSR before VMENTRY, and control which mitigations the guest can enable. In the case of SSBD, unless the host has enabled SSBD always on mode (by passing "spec_store_bypass_disable=on" in the kernel parameters), the SSBD bit is not set in the mask and the guest can not properly enable the SSBD always on mitigation mode. This has been confirmed by running the SSBD PoC on a guest using the SSBD always on mitigation mode (booted with kernel parameter "spec_store_bypass_disable=on"), and verifying that the guest is vulnerable unless the host is also using SSBD always on mode. In addition, the guest OS incorrectly reports the SSB vulnerability as mitigated. Always set the SSBD bit in x86_spec_ctrl_mask when the host CPU supports it, allowing the guest to use SSBD whether or not the host has chosen to enable the mitigation in any of its modes. Fixes:be6fcb5478("x86/bugs: Rework spec_ctrl base and mask logic") Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Reviewed-by: Liam Merwick <liam.merwick@oracle.com> Reviewed-by: Mark Kanda <mark.kanda@oracle.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Cc: bp@alien8.de Cc: rkrcmar@redhat.com Cc: kvm@vger.kernel.org Cc: stable@vger.kernel.org Link: https://lkml.kernel.org/r/1560187210-11054-1-git-send-email-alejandro.j.jimenez@oracle.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. 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.