[PATCH 4.14 032/164] x86/xen: Delay get_cpu_cap until stack canary is established

From: Greg Kroah-Hartman
Date: Sun Apr 22 2018 - 11:33:19 EST

4.14-stable review patch. If anyone has any objections, please let me know.


From: Jason Andryuk <jandryuk@xxxxxxxxx>

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@xxxxxxxxx>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

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
@@ -1258,10 +1258,6 @@ 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 */

@@ -1271,6 +1267,10 @@ asmlinkage __visible void __init xen_sta

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

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