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

tip-bot for Sebastian Andrzej Siewior
2019-04-13T20:51:19Z

Commit-ID: 39388e80f9b0c3788bfb6efe3054bdce0c3ead45
Gitweb: https://git.kernel.org/tip/39388e80f9b0c3788bfb6efe3054bdce0c3ead45
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate: Wed, 3 Apr 2019 18:41:35 +0200
Committer: Borislav Petkov <bp@suse.de>
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@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc: kvm ML <kvm@vger.kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20190403164156.19645-7-bigeasy@linutronix.de
---
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))