Patch "x86/fpu: Fix invalid FPU ptrace state after execve()" has been added to the 4.8-stable tree

This is a note to let you know that I've just added the patch titled

x86/fpu: Fix invalid FPU ptrace state after execve()

to the 4.8-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-fpu-fix-invalid-fpu-ptrace-state-after-execve.patch
and it can be found in the queue-4.8 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Yu-cheng Yu <[email protected]>
Date: Thu, 17 Nov 2016 09:11:35 -0800
Subject: x86/fpu: Fix invalid FPU ptrace state after execve()

From: Yu-cheng Yu <[email protected]>

commit b22cbe404a9cc3c7949e380fa1861e31934c8978 upstream.

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.

---
 arch/x86/kernel/fpu/core.c |   16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

@@ -517,14 +517,14 @@ void fpu__clear(struct fpu *fpu)
 {
 	WARN_ON_FPU(fpu != &current->thread.fpu); /* Almost certainly an anomaly */
 
-	if (!use_eager_fpu() || !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();
 	}
 }

Patches currently in stable-queue which might be from [email protected] are

queue-4.8/x86-fpu-fix-invalid-fpu-ptrace-state-after-execve.patch