Reload CS when startup_64 is used. From: ebiederm@xmission.com (Eric W. Biederman) In long mode the %cs is largely a relic. However there are a few cases like iret where it matters that we have a valid value. Without this patch it is possible to enter the kernel in startup_64 without setting %cs to a valid value. With this patch we don't care what %cs value we enter the kernel with, so long as the cs shadow register indicates it is a privileged code segment. Thanks to Magnus Damm for finding this problem and posting the first workable patch. I have moved the jump to set %cs down a few instructions so we don't need to take an extra jump. Which keeps the code simpler. Signed-of-by: Eric W. Biederman Signed-off-by: Andi Kleen --- arch/x86_64/kernel/head.S | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) Index: linux/arch/x86_64/kernel/head.S =================================================================== --- linux.orig/arch/x86_64/kernel/head.S +++ linux/arch/x86_64/kernel/head.S @@ -185,12 +185,15 @@ startup_64: /* Finally jump to run C code and to be on real kernel address * Since we are running on identity-mapped space we have to jump - * to the full 64bit address , this is only possible as indirect - * jump + * to the full 64bit address, this is only possible as indirect + * jump. In addition we need to ensure %cs is set so we make this + * a far return. */ movq initial_code(%rip),%rax - pushq $0 # fake return address - jmp *%rax + pushq $0 # fake return address to stop unwinder + pushq $__KERNEL_CS # set correct cs + pushq %rax # target address in negative space + lretq /* SMP bootup changes these two */ .align 8