[Xen-devel] [PATCH] x86/xen: Delay get_cpu_cap until stack canary is established

[Jason Andryuk <jandryuk@xxxxxxxxx>]

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

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 3c2c2530737e..c36d23aa6c35 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/arch/x86/xen/enlighten_pv.c
@@ -1259,10 +1259,6 @@ asmlinkage __visible void __init xen_start_kernel(void)
         */
        __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_start_kernel(void)
         */
        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. */
-- 
2.14.3


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