[PATCH v2] x86/fpu: Fix invalid FPU ptrace state after execve

[Yu-cheng Yu <yu-cheng.yu@intel.com>]

Version 2 is based on tip/master.  The earlier version was based on the
upstream tree, but essentially the same patch.

Robert O'Callahan reported that after an execve PTRACE_GETREGSET
NT_X86_XSTATE continues to return the pre-exec register values
until the exec'ed task modifies FPU state.  The test code is at
https://bugzilla.redhat.com/attachment.cgi?id=1164286.

What is happening is fpu__clear() does not properly clear fpstate.
Fix it by doing just that.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reported-by: Robert O'Callahan <robert@ocallahan.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Robert O'Callahan <robert@ocallahan.org>
Cc: "Ravi V. Shankar" <ravi.v.shankar@intel.com>
---
 arch/x86/kernel/fpu/core.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 7d8e262..e4e97a5 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -461,14 +461,14 @@ void fpu__clear(struct fpu *fpu)
 {
 	WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */
 
-	if (!static_cpu_has(X86_FEATURE_FPU)) {
-		/* FPU state will be reallocated lazily at the first use. */
-		fpu__drop(fpu);
-	} else {
-		if (!fpu->fpstate_active) {
-			fpu__activate_curr(fpu);
-			user_fpu_begin();
-		}
+	fpu__drop(fpu);
+
+	/*
+	 * Make sure fpstate is cleared and initialized.
+	 */
+	if (static_cpu_has(X86_FEATURE_FPU)) {
+		fpu__activate_curr(fpu);
+		user_fpu_begin();
 		copy_init_fpstate_to_fpregs();
 	}
 }
-- 
1.9.1