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    Subject[PATCH v12 13/18] x86/fsgsbase/64: Use FSGSBASE instructions on thread copy and ptrace
    Date
    From: "Chang S. Bae" <chang.seok.bae@intel.com>

    When FSGSBASE is enabled, copying threads and reading FS/GS base using
    ptrace must read the actual values.

    When copying a thread, use fsgs_save() and copy the saved values. For
    ptrace, the bases must be read from memory regardless of the selector
    if FSGSBASE is enabled.

    Suggested-by: Andy Lutomirski <luto@kernel.org>
    Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    Reviewed-by: Tony Luck <tony.luck@intel.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Borislav Petkov <bp@alien8.de>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: H. Peter Anvin <hpa@zytor.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Andi Kleen <ak@linux.intel.com>
    ---
    arch/x86/kernel/process.c | 9 +++++----
    arch/x86/kernel/process_64.c | 6 ++++--
    2 files changed, 9 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    index 9da70b279dad8..31dd24f9c8d8e 100644
    --- a/arch/x86/kernel/process.c
    +++ b/arch/x86/kernel/process.c
    @@ -140,10 +140,11 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
    memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));

    #ifdef CONFIG_X86_64
    - savesegment(gs, p->thread.gsindex);
    - p->thread.gsbase = p->thread.gsindex ? 0 : current->thread.gsbase;
    - savesegment(fs, p->thread.fsindex);
    - p->thread.fsbase = p->thread.fsindex ? 0 : current->thread.fsbase;
    + save_fsgs(current);
    + p->thread.fsindex = current->thread.fsindex;
    + p->thread.fsbase = current->thread.fsbase;
    + p->thread.gsindex = current->thread.gsindex;
    + p->thread.gsbase = current->thread.gsbase;
    savesegment(es, p->thread.es);
    savesegment(ds, p->thread.ds);
    #else
    diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
    index 4be88124d81ea..57cdbbb0381ac 100644
    --- a/arch/x86/kernel/process_64.c
    +++ b/arch/x86/kernel/process_64.c
    @@ -346,7 +346,8 @@ unsigned long x86_fsbase_read_task(struct task_struct *task)

    if (task == current)
    fsbase = x86_fsbase_read_cpu();
    - else if (task->thread.fsindex == 0)
    + else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
    + (task->thread.fsindex == 0))
    fsbase = task->thread.fsbase;
    else
    fsbase = x86_fsgsbase_read_task(task, task->thread.fsindex);
    @@ -360,7 +361,8 @@ unsigned long x86_gsbase_read_task(struct task_struct *task)

    if (task == current)
    gsbase = x86_gsbase_read_cpu_inactive();
    - else if (task->thread.gsindex == 0)
    + else if (static_cpu_has(X86_FEATURE_FSGSBASE) ||
    + (task->thread.gsindex == 0))
    gsbase = task->thread.gsbase;
    else
    gsbase = x86_fsgsbase_read_task(task, task->thread.gsindex);
    --
    2.20.1
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