linux-kernel - [patch 09/31] x86/fpu: Do not inherit FPU context for CLONE

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Date:   Tue, 12 Oct 2021 02:00:11 +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>
Subject: [patch 09/31] x86/fpu: Do not inherit FPU context for CLONE_THREAD

CLONE_THREAD does not have the guarantee of a true fork to inherit all
state. Especially the FPU state is meaningless for CLONE_THREAD.

Just wipe out the minimal required state so restore on return to user space
let's the thread start with a clean FPU.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 arch/x86/include/asm/fpu/internal.h |    2 +-
 arch/x86/kernel/fpu/core.c          |    8 +++++---
 arch/x86/kernel/process.c           |    2 +-
 3 files changed, 7 insertions(+), 5 deletions(-)

--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -34,7 +34,7 @@ extern int  fpu__exception_code(struct f
 extern void fpu_sync_fpstate(struct fpu *fpu);
 
 /* Clone and exit operations */
-extern int  fpu_clone(struct task_struct *dst);
+extern int  fpu_clone(struct task_struct *dst, unsigned long clone_flags);
 extern void fpu_flush_thread(void);
 
 /*
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -257,7 +257,7 @@ void fpstate_init(union fpregs_state *st
 EXPORT_SYMBOL_GPL(fpstate_init);
 
 /* Clone current's FPU state on fork */
-int fpu_clone(struct task_struct *dst)
+int fpu_clone(struct task_struct *dst, unsigned long clone_flags)
 {
 	struct fpu *src_fpu = &current->thread.fpu;
 	struct fpu *dst_fpu = &dst->thread.fpu;
@@ -276,9 +276,11 @@ int fpu_clone(struct task_struct *dst)
 
 	/*
 	 * No FPU state inheritance for kernel threads and IO
-	 * worker threads.
+	 * worker threads. Neither CLONE_THREAD needs a copy
+	 * of the FPU state.
 	 */
-	if (dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
+	if (clone_flags & CLONE_THREAD ||
+	    dst->flags & (PF_KTHREAD | PF_IO_WORKER)) {
 		/* Clear out the minimal state */
 		memcpy(&dst_fpu->state, &init_fpstate,
 		       init_fpstate_copy_size());
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -154,7 +154,7 @@ int copy_thread(unsigned long clone_flag
 	frame->flags = X86_EFLAGS_FIXED;
 #endif
 
-	fpu_clone(p);
+	fpu_clone(p, clone_flags);
 
 	/* Kernel thread ? */
 	if (unlikely(p->flags & PF_KTHREAD)) {