[PATCH] x86/fpu: Update kernel's FPU state before using for the fsave header

[Sebastian Andrzej Siewior]

In commit

  39388e80f9b0c ("x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe()")

I removed the statement
|       if (ia32_fxstate)
|               copy_fxregs_to_kernel(fpu);

and argued that is was wrongly merged because the content was already
saved in kernel's state and the content.
This was wrong: It is required to write it back because it is only saved
on the user-stack and save_fsave_header() reads it from task's
FPU-state. I missed that partâ

Save x87 FPU state unless thread's FPU registers are already up to date.

Fixes: 39388e80f9b0c ("x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe()")
Reported-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 arch/x86/kernel/fpu/signal.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index 060d6188b4533..0071b794ed193 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -62,6 +62,11 @@ static inline int save_fsave_header(struct task_struct *tsk, void __user *buf)
 		struct user_i387_ia32_struct env;
 		struct _fpstate_32 __user *fp = buf;
 
+		fpregs_lock();
+		if (!test_thread_flag(TIF_NEED_FPU_LOAD))
+			copy_fxregs_to_kernel(&tsk->thread.fpu);
+		fpregs_unlock();
+
 		convert_from_fxsr(&env, tsk);
 
 		if (__copy_to_user(buf, &env, sizeof(env)) ||
-- 
2.20.1