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    Subject[PATCH 4.14 038/167] x86/xen: init %gs very early to avoid page faults with stack protector
    Date
    4.14-stable review patch.  If anyone has any objections, please let me know.

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

    From: Juergen Gross <jgross@suse.com>

    commit 4f277295e54c5b7340e48efea3fc5cc21a2872b7 upstream.

    When running as Xen pv guest %gs is initialized some time after
    C code is started. Depending on stack protector usage this might be
    too late, resulting in page faults.

    So setup %gs and MSR_GS_BASE in assembly code already.

    Cc: stable@vger.kernel.org
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Tested-by: Chris Patterson <cjp256@gmail.com>
    Signed-off-by: Juergen Gross <jgross@suse.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/xen/xen-head.S | 16 ++++++++++++++++
    1 file changed, 16 insertions(+)

    --- a/arch/x86/xen/xen-head.S
    +++ b/arch/x86/xen/xen-head.S
    @@ -9,7 +9,9 @@

    #include <asm/boot.h>
    #include <asm/asm.h>
    +#include <asm/msr.h>
    #include <asm/page_types.h>
    +#include <asm/percpu.h>
    #include <asm/unwind_hints.h>

    #include <xen/interface/elfnote.h>
    @@ -35,6 +37,20 @@ ENTRY(startup_xen)
    mov %_ASM_SI, xen_start_info
    mov $init_thread_union+THREAD_SIZE, %_ASM_SP

    +#ifdef CONFIG_X86_64
    + /* Set up %gs.
    + *
    + * The base of %gs always points to the bottom of the irqstack
    + * union. If the stack protector canary is enabled, it is
    + * located at %gs:40. Note that, on SMP, the boot cpu uses
    + * init data section till per cpu areas are set up.
    + */
    + movl $MSR_GS_BASE,%ecx
    + movq $INIT_PER_CPU_VAR(irq_stack_union),%rax
    + cdq
    + wrmsr
    +#endif
    +
    jmp xen_start_kernel
    END(startup_xen)
    __FINIT

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