[PATCH 12/15] x86/fsgsbase/64: When copying a thread, use FSGSBASE if enabled

[Chang S. Bae]

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....@intel.com>
Cc: Andy Lutomirski <l...@kernel.org>
Cc: H. Peter Anvin <h...@zytor.com>
Cc: Andi Kleen <a...@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