linux-kernel - [tip:x86/urgent] x86/fpu: Don't use current->mm to check for a kthread

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Date:   Thu, 13 Jun 2019 12:07:13 -0700
From:   tip-bot for Christoph Hellwig <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     bigeasy@...utronix.de, hch@....de, jannh@...gle.com, bp@...e.de,
        mingo@...hat.com, aubrey.li@...el.com, dave.hansen@...el.com,
        linux-kernel@...r.kernel.org, hpa@...or.com, x86@...nel.org,
        tglx@...utronix.de, riel@...riel.com, mingo@...nel.org,
        nstange@...e.de, peterz@...radead.org
Subject: [tip:x86/urgent] x86/fpu: Don't use current->mm to check for a
 kthread

Commit-ID:  8d3289f2fa1e0c7e2f72c7352f1efb75d2ad7c76
Gitweb:     https://git.kernel.org/tip/8d3289f2fa1e0c7e2f72c7352f1efb75d2ad7c76
Author:     Christoph Hellwig <hch@....de>
AuthorDate: Tue, 4 Jun 2019 19:54:12 +0200
Committer:  Borislav Petkov <bp@...e.de>
CommitDate: Thu, 13 Jun 2019 20:57:49 +0200

x86/fpu: Don't use current->mm to check for a kthread

current->mm can be non-NULL if a kthread calls use_mm(). Check for
PF_KTHREAD instead to decide when to store user mode FP state.

Fixes: 2722146eb784 ("x86/fpu: Remove fpu->initialized")
Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Christoph Hellwig <hch@....de>
Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Aubrey Li <aubrey.li@...el.com>
Cc: Dave Hansen <dave.hansen@...el.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Jann Horn <jannh@...gle.com>
Cc: Nicolai Stange <nstange@...e.de>
Cc: Rik van Riel <riel@...riel.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: x86-ml <x86@...nel.org>
Link: https://lkml.kernel.org/r/20190604175411.GA27477@lst.de
---
 arch/x86/include/asm/fpu/internal.h | 6 +++---
 arch/x86/kernel/fpu/core.c          | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index 9e27fa05a7ae..4c95c365058a 100644
--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -536,7 +536,7 @@ static inline void __fpregs_load_activate(void)
 	struct fpu *fpu = &current->thread.fpu;
 	int cpu = smp_processor_id();
 
-	if (WARN_ON_ONCE(current->mm == NULL))
+	if (WARN_ON_ONCE(current->flags & PF_KTHREAD))
 		return;
 
 	if (!fpregs_state_valid(fpu, cpu)) {
@@ -567,11 +567,11 @@ static inline void __fpregs_load_activate(void)
  * otherwise.
  *
  * The FPU context is only stored/restored for a user task and
- * ->mm is used to distinguish between kernel and user threads.
+ * PF_KTHREAD is used to distinguish between kernel and user threads.
  */
 static inline void switch_fpu_prepare(struct fpu *old_fpu, int cpu)
 {
-	if (static_cpu_has(X86_FEATURE_FPU) && current->mm) {
+	if (static_cpu_has(X86_FEATURE_FPU) && !(current->flags & PF_KTHREAD)) {
 		if (!copy_fpregs_to_fpstate(old_fpu))
 			old_fpu->last_cpu = -1;
 		else
diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 466fca686fb9..649fbc3fcf9f 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -102,7 +102,7 @@ static void __kernel_fpu_begin(void)
 
 	kernel_fpu_disable();
 
-	if (current->mm) {
+	if (!(current->flags & PF_KTHREAD)) {
 		if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
 			set_thread_flag(TIF_NEED_FPU_LOAD);
 			/*