linux-kernel - [tip: x86/fpu] x86/fpu: Remove pointless memset in fpu

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Message-ID: <163473748299.25758.1764135013570035920.tip-bot2@tip-bot2>
Date:   Wed, 20 Oct 2021 13:44:42 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...e.de>,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/fpu] x86/fpu: Remove pointless memset in fpu_clone()

The following commit has been merged into the x86/fpu branch of tip:

Commit-ID:     01f9f62d3ae75077a54a11d2777082f1e58e2d9f
Gitweb:        https://git.kernel.org/tip/01f9f62d3ae75077a54a11d2777082f1e58e2d9f
Author:        Thomas Gleixner <tglx@...utronix.de>
AuthorDate:    Fri, 15 Oct 2021 03:16:02 +02:00
Committer:     Borislav Petkov <bp@...e.de>
CommitterDate: Wed, 20 Oct 2021 15:27:26 +02:00

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: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Borislav Petkov <bp@...e.de>
Link: https://lkml.kernel.org/r/20211015011538.722854569@linutronix.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 7ada7bd..191269e 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.