x86/doublefault/32: Fix stack canaries in the double fault handler

commit 3580d0b29cab08483f84a16ce6a1151a1013695f upstream.

The double fault TSS was missing GS setup, which is needed for stack
canaries to work.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/kernel/doublefault.c

@@ -65,6 +65,9 @@ struct x86_hw_tss doublefault_tss __cacheline_aligned = {
 	.ss		= __KERNEL_DS,
 	.ds		= __USER_DS,
 	.fs		= __KERNEL_PERCPU,
+#ifndef CONFIG_X86_32_LAZY_GS
+	.gs		= __KERNEL_STACK_CANARY,
+#endif
 
 	.__cr3		= __pa_nodebug(swapper_pg_dir),
 };