linux-kernel - [patch V2 06/30] x86/fpu: Remove pointless memset in fpu

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Date:   Fri, 15 Oct 2021 03:16:02 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>
Subject: [patch V2 06/30] x86/fpu: Remove pointless memset in fpu_clone()

Zeroing the forked task's FPU register 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
register 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 nowhere used and cannot leak anywhere.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

---
 arch/x86/kernel/fpu/core.c | 6 ------
 1 file changed, 6 deletions(-)
---
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 7ada7bd03a32..191269edac97 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -260,12 +260,6 @@ int fpu_clone(struct task_struct *dst)
 		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
 	 * child's FPU context, without any memory-to-memory copying.