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    Subject[PATCH 12/15] x86/fsgsbase/64: When copying a thread, use FSGSBASE if enabled
    Date
    When FSGSBASE enabled, copy real FS/GS base values instead
    of approximation.

    Factor out to save_fsgs() does not yield the exact same
    behavior, because save_base_legacy() does not copy FS/GS base
    when index is zero.

    Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Andi Kleen <ak@linux.intel.com>
    ---
    arch/x86/kernel/process_64.c | 12 +++++++++---
    1 file changed, 9 insertions(+), 3 deletions(-)

    diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
    index 7249a54..5aae132 100644
    --- a/arch/x86/kernel/process_64.c
    +++ b/arch/x86/kernel/process_64.c
    @@ -491,10 +491,16 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
    p->thread.sp = (unsigned long) fork_frame;
    p->thread.io_bitmap_ptr = NULL;

    - savesegment(gs, p->thread.gsindex);
    - p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
    savesegment(fs, p->thread.fsindex);
    - p->thread.fsbase = p->thread.fsindex ? 0 : me->thread.fsbase;
    + savesegment(gs, p->thread.gsindex);
    + if (static_cpu_has(X86_FEATURE_FSGSBASE)) {
    + p->thread.fsbase = rdfsbase();
    + p->thread.gsbase = rd_shadow_gsbase();
    + } else {
    + /* save_base_legacy() does not set base when index is zero. */
    + p->thread.fsbase = p->thread.fsindex ? 0 : me->thread.fsbase;
    + p->thread.gsbase = p->thread.gsindex ? 0 : me->thread.gsbase;
    + }
    savesegment(es, p->thread.es);
    savesegment(ds, p->thread.ds);
    memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
    --
    2.7.4
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