x86/fpu: Remove pointless memset in fpu_clone()

mainline inclusion
from mainline-v5.16-rc1
commit 01f9f62d
category: feature
bugzilla: https://gitee.com/openeuler/intel-kernel/issues/I590ZC
CVE: NA

Intel-SIG: commit 01f9f62d x86/fpu: Remove pointless memset in fpu_clone().

--------------------------------

Zeroing the forked task's FPU registers buffer to avoid leaking init
optimized stale data into the clone is a pointless exercise for the case
where the current task has TIF_NEED_FPU_LOAD set. In that case, the FPU
registers state is copied from current's FPU register buffer which can
contain stale init optimized data as well.

The alledged information leak is non-existant because this stale init
optimized data is used nowhere and cannot leak anywhere.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NBorislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.722854569@linutronix.deSigned-off-by: NLin Wang <lin.x.wang@intel.com>
Signed-off-by: NAichun Shi <aichun.shi@intel.com>

arch/x86/kernel/fpu/core.c

@@ -259,12 +259,6 @@ int fpu_clone(struct task_struct *dst)
 	if (!cpu_feature_enabled(X86_FEATURE_FPU))
 		return 0;
 
-	/*
-	 * Don't let 'init optimized' areas of the XSAVE area
-	 * leak into the child task:
-	 */
-	memset(&dst_fpu->state.xsave, 0, fpu_kernel_xstate_size);
-
 	/*
 	 * If the FPU registers are not owned by current just memcpy() the
 	 * state.  Otherwise save the FPU registers directly into the