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

[tip-bot for Sebastian Andrzej Siewior]

Commit-ID:  aab8445c4e1cceeb3f739352041ec1c2586bc923
Gitweb:     https://git.kernel.org/tip/aab8445c4e1cceeb3f739352041ec1c2586bc923
Author:     Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate: Fri, 7 Jun 2019 16:29:16 +0200
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Sat, 8 Jun 2019 11:45:15 +0200

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

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 it was wrongly merged because the content was already
saved in kernel's state.

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>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Tested-by: Eric Biggers <ebiggers@xxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
Cc: kvm ML <kvm@xxxxxxxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190607142915.y52mfmgk5lvhll7n@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 060d6188b453..0071b794ed19 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)) ||