x86/fpu: Deactivate FPU state after failure during state load

In __fpu__restore_sig(), fpu_fpregs_owner_ctx needs to be reset if the
FPU state was not fully restored. Otherwise the following may happen (on
the same CPU):

  Task A                     Task B               fpu_fpregs_owner_ctx
  *active*                                        A.fpu
  __fpu__restore_sig()
                             ctx switch           load B.fpu
                             *active*             B.fpu
  fpregs_lock()
  copy_user_to_fpregs_zeroing()
    copy_kernel_to_xregs() *modify*
    copy_user_to_xregs() *fails*
  fpregs_unlock()
                            ctx switch            skip loading B.fpu,
                            *active*              B.fpu

In the success case, fpu_fpregs_owner_ctx is set to the current task.

In the failure case, the FPU state might have been modified by loading
the init state.

In this case, fpu_fpregs_owner_ctx needs to be reset in order to ensure
that the FPU state of the following task is loaded from saved state (and
not skipped because it was the previous state).

Reset fpu_fpregs_owner_ctx after a failure during restore occurred, to
ensure that the FPU state for the next task is always loaded.

The problem was debugged-by Yu-cheng Yu <[email protected]>.

 [ bp: Massage commit message. ]

Fixes: 5f409e2 ("x86/fpu: Defer FPU state load until return to userspace")
Reported-by: Yu-cheng Yu <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: "Ravi V. Shankar" <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: Sebastian Andrzej Siewior

arch/x86/kernel/fpu/signal.c

@@ -352,6 +352,7 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 			fpregs_unlock();
 			return 0;
 		}
+		fpregs_deactivate(fpu);
 		fpregs_unlock();
 	}
 
@@ -403,6 +404,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
 	}
 	if (!ret)
 		fpregs_mark_activate();
+	else
+		fpregs_deactivate(fpu);
 	fpregs_unlock();
 
 err_out: