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    Subject[PATCH 4.4 052/107] x86/bugs, KVM: Support the combination of guest and host IBRS
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

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

    From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

    commit 5cf687548705412da47c9cec342fd952d71ed3d5 upstream

    A guest may modify the SPEC_CTRL MSR from the value used by the
    kernel. Since the kernel doesn't use IBRS, this means a value of zero is
    what is needed in the host.

    But the 336996-Speculative-Execution-Side-Channel-Mitigations.pdf refers to
    the other bits as reserved so the kernel should respect the boot time
    SPEC_CTRL value and use that.

    This allows to deal with future extensions to the SPEC_CTRL interface if
    any at all.

    Note: This uses wrmsrl() instead of native_wrmsl(). I does not make any
    difference as paravirt will over-write the callq *0xfff.. with the wrmsrl
    assembler code.

    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Reviewed-by: Borislav Petkov <bp@suse.de>
    Reviewed-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    [ Srivatsa: Backported to 4.4.y, skipping the KVM changes in this patch. ]
    Signed-off-by: Srivatsa S. Bhat <srivatsa@csail.mit.edu>
    Reviewed-by: Matt Helsley (VMware) <matt.helsley@gmail.com>
    Reviewed-by: Alexey Makhalov <amakhalov@vmware.com>
    Reviewed-by: Bo Gan <ganb@vmware.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---

    arch/x86/include/asm/nospec-branch.h | 10 ++++++++++
    arch/x86/kernel/cpu/bugs.c | 18 ++++++++++++++++++
    2 files changed, 28 insertions(+)

    --- a/arch/x86/include/asm/nospec-branch.h
    +++ b/arch/x86/include/asm/nospec-branch.h
    @@ -183,6 +183,16 @@ enum spectre_v2_mitigation {
    extern void x86_spec_ctrl_set(u64);
    extern u64 x86_spec_ctrl_get_default(void);

    +/*
    + * On VMENTER we must preserve whatever view of the SPEC_CTRL MSR
    + * the guest has, while on VMEXIT we restore the host view. This
    + * would be easier if SPEC_CTRL were architecturally maskable or
    + * shadowable for guests but this is not (currently) the case.
    + * Takes the guest view of SPEC_CTRL MSR as a parameter.
    + */
    +extern void x86_spec_ctrl_set_guest(u64);
    +extern void x86_spec_ctrl_restore_host(u64);
    +
    extern char __indirect_thunk_start[];
    extern char __indirect_thunk_end[];

    --- a/arch/x86/kernel/cpu/bugs.c
    +++ b/arch/x86/kernel/cpu/bugs.c
    @@ -122,6 +122,24 @@ u64 x86_spec_ctrl_get_default(void)
    }
    EXPORT_SYMBOL_GPL(x86_spec_ctrl_get_default);

    +void x86_spec_ctrl_set_guest(u64 guest_spec_ctrl)
    +{
    + if (!boot_cpu_has(X86_FEATURE_IBRS))
    + return;
    + if (x86_spec_ctrl_base != guest_spec_ctrl)
    + wrmsrl(MSR_IA32_SPEC_CTRL, guest_spec_ctrl);
    +}
    +EXPORT_SYMBOL_GPL(x86_spec_ctrl_set_guest);
    +
    +void x86_spec_ctrl_restore_host(u64 guest_spec_ctrl)
    +{
    + if (!boot_cpu_has(X86_FEATURE_IBRS))
    + return;
    + if (x86_spec_ctrl_base != guest_spec_ctrl)
    + wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
    +}
    +EXPORT_SYMBOL_GPL(x86_spec_ctrl_restore_host);
    +
    #ifdef RETPOLINE
    static bool spectre_v2_bad_module;


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