linux-kernel - [v3 06/12] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available

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Date:   Tue, 23 Oct 2018 11:42:28 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Markus T Metzger <markus.t.metzger@...el.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        "Chang S . Bae" <chang.seok.bae@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: [v3 06/12] x86/fsgsbase/64: When copying a thread, use the FSGSBASE instructions if available

Copy real FS/GSBASE values instead of approximation when FSGSBASE is
enabled.

Factoring out to save_fsgs() does not result in the same behavior because
save_base_legacy() does not copy FS/GSBASE when the index is zero.

Signed-off-by: Chang S. Bae <chang.seok.bae@...el.com>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: H. Peter Anvin <hpa@...or.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Andi Kleen <ak@...ux.intel.com>
Cc: Dave Hansen <dave.hansen@...ux.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 1d975cadc256..71cc8abe208c 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -510,10 +510,16 @@ 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_inactive_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));
-- 
2.19.1