[tip:x86/fpu] x86/fpu: Don't save fxregs for ia32 frames in copy_fpstate_to_sigframe()

[tip-bot for Sebastian Andrzej Siewior <tipbot@xxxxxxxxx>]

Commit-ID:  39388e80f9b0c3788bfb6efe3054bdce0c3ead45
Gitweb:     https://git.kernel.org/tip/39388e80f9b0c3788bfb6efe3054bdce0c3ead45
Author:     Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
AuthorDate: Wed, 3 Apr 2019 18:41:35 +0200
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Wed, 10 Apr 2019 14:46:35 +0200

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

In commit

  72a671ced66db ("x86, fpu: Unify signal handling code paths for x86 and x86_64 kernels")

the 32bit and 64bit path of the signal delivery code were merged.

The 32bit version:

  int save_i387_xstate_ia32(void __user *buf)
  …
         if (cpu_has_xsave)
                 return save_i387_xsave(fp);
         if (cpu_has_fxsr)
                 return save_i387_fxsave(fp);

The 64bit version:

  int save_i387_xstate(void __user *buf)
  …
         if (user_has_fpu()) {
                 if (use_xsave())
                         err = xsave_user(buf);
                 else
                         err = fxsave_user(buf);

                 if (unlikely(err)) {
                         __clear_user(buf, xstate_size);
                         return err;

The merge:

  int save_xstate_sig(void __user *buf, void __user *buf_fx, int size)
  …
         if (user_has_fpu()) {
                 /* Save the live register state to the user directly. */
                 if (save_user_xstate(buf_fx))
                         return -1;
                 /* Update the thread's fxstate to save the fsave header. */
                 if (ia32_fxstate)
                         fpu_fxsave(&tsk->thread.fpu);

I don't think that we needed to save the FPU registers to ->thread.fpu
because the registers were stored in buf_fx. Today the state will be
restored from buf_fx after the signal was handled (I assume that this
was also the case with lazy-FPU).

Since commit

  66463db4fc560 ("x86, fpu: shift drop_init_fpu() from save_xstate_sig() to handle_signal()")

it is ensured that the signal handler starts with clear/fresh set of FPU
registers which means that the previous store is futile.

Remove the copy_fxregs_to_kernel() call because task's FPU state is
cleared later in handle_signal() via fpu__clear().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
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 Krčmář <rkrcmar@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/20190403164156.19645-7-bigeasy@xxxxxxxxxxxxx
---
 arch/x86/kernel/fpu/signal.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
index c1a5999affa0..34989d2a8893 100644
--- a/arch/x86/kernel/fpu/signal.c
+++ b/arch/x86/kernel/fpu/signal.c
@@ -155,7 +155,6 @@ static inline int copy_fpregs_to_sigframe(struct xregs_state __user *buf)
  */
 int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 {
-	struct fpu *fpu = &current->thread.fpu;
 	struct task_struct *tsk = current;
 	int ia32_fxstate = (buf != buf_fx);
 
@@ -173,9 +172,6 @@ int copy_fpstate_to_sigframe(void __user *buf, void __user *buf_fx, int size)
 	/* Save the live registers state to the user frame directly. */
 	if (copy_fpregs_to_sigframe(buf_fx))
 		return -1;
-	/* Update the thread's fxstate to save the fsave header. */
-	if (ia32_fxstate)
-		copy_fxregs_to_kernel(fpu);
 
 	/* Save the fsave header for the 32-bit frames. */
 	if ((ia32_fxstate || !use_fxsr()) && save_fsave_header(tsk, buf))