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    Subject[PATCH 4.16 035/196] x86/xen: Delay get_cpu_cap until stack canary is established
    Date
    4.16-stable review patch.  If anyone has any objections, please let me know.

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

    From: Jason Andryuk <jandryuk@gmail.com>

    commit 36104cb9012a82e73c32a3b709257766b16bcd1d upstream.

    Commit 2cc42bac1c79 ("x86-64/Xen: eliminate W+X mappings") introduced a
    call to get_cpu_cap, which is fstack-protected. This is works on x86-64
    as commit 4f277295e54c ("x86/xen: init %gs very early to avoid page
    faults with stack protector") ensures the stack protector is configured,
    but it it did not cover x86-32.

    Delay calling get_cpu_cap until after xen_setup_gdt has initialized the
    stack canary. Without this, a 32bit PV machine crashes early
    in boot.
    (XEN) Domain 0 (vcpu#0) crashed on cpu#0:
    (XEN) ----[ Xen-4.6.6-xc x86_64 debug=n Tainted: C ]----
    (XEN) CPU: 0
    (XEN) RIP: e019:[<00000000c10362f8>]

    And the PV kernel IP corresponds to init_scattered_cpuid_features
    0xc10362f8 <+24>: mov %gs:0x14,%eax

    Fixes 2cc42bac1c79 ("x86-64/Xen: eliminate W+X mappings")

    Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
    Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/xen/enlighten_pv.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- a/arch/x86/xen/enlighten_pv.c
    +++ b/arch/x86/xen/enlighten_pv.c
    @@ -1259,10 +1259,6 @@ asmlinkage __visible void __init xen_sta
    */
    __userpte_alloc_gfp &= ~__GFP_HIGHMEM;

    - /* Work out if we support NX */
    - get_cpu_cap(&boot_cpu_data);
    - x86_configure_nx();
    -
    /* Get mfn list */
    xen_build_dynamic_phys_to_machine();

    @@ -1272,6 +1268,10 @@ asmlinkage __visible void __init xen_sta
    */
    xen_setup_gdt(0);

    + /* Work out if we support NX */
    + get_cpu_cap(&boot_cpu_data);
    + x86_configure_nx();
    +
    xen_init_irq_ops();

    /* Let's presume PV guests always boot on vCPU with id 0. */

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