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    Subject[PATCH] x86_64: Reload CS when startup_64 is used.

    In long mode the %cs is largely a relic. However there are a few cases
    like lret 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 <>
    arch/x86_64/kernel/head.S | 7 +++++--
    1 files changed, 5 insertions(+), 2 deletions(-)

    diff --git a/arch/x86_64/kernel/head.S b/arch/x86_64/kernel/head.S
    index 923f080..6c89f6a 100644
    --- a/arch/x86_64/kernel/head.S
    +++ b/arch/x86_64/kernel/head.S
    @@ -186,10 +186,13 @@ #define CR0_PAGING (1<<31)
    /* 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
    + * jump. In addition we need to ensure %cs is set so we make this
    + * a far return.
    movq initial_code(%rip),%rax
    - jmp *%rax
    + pushq $__KERNEL_CS
    + pushq %rax
    + lretq

    /* SMP bootup changes these two */
    .align 8
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      Last update: 2006-08-22 10:41    [W:0.020 / U:192.144 seconds]
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