[patch V3 3/6] x86/process: Check PF_KTHREAD and not current->mm for kernel threads

From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: x86@kernel.org, Andy Lutomirski <luto@kernel.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Fenghua Yu <fenghua.yu@intel.com>,
	Tony Luck <tony.luck@intel.com>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Rik van Riel <riel@surriel.com>, Borislav Petkov <bp@suse.de>
Subject: [patch V3 3/6] x86/process: Check PF_KTHREAD and not current->mm for kernel threads
Date: Tue, 08 Jun 2021 16:36:20 +0200	[thread overview]
Message-ID: <20210608144345.912645927@linutronix.de> (raw)
In-Reply-To: 20210608143617.565868844@linutronix.de

switch_fpu_finish() checks current->mm as indicator for kernel threads.
That's wrong because kernel threads can temporarily use a mm of a user
process via kthread_use_mm().

Check the task flags for PF_KTHREAD instead.

Fixes: 0cecca9d03c9 ("x86/fpu: Eager switch PKRU state")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Rik van Riel <riel@surriel.com>
Cc: stable@vger.kernel.org
---
 arch/x86/include/asm/fpu/internal.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/include/asm/fpu/internal.h
+++ b/arch/x86/include/asm/fpu/internal.h
@@ -578,7 +578,7 @@ static inline void switch_fpu_finish(str
 	 * PKRU state is switched eagerly because it needs to be valid before we
 	 * return to userland e.g. for a copy_to_user() operation.
 	 */
-	if (current->mm) {
+	if (!(current->flags & PF_KTHREAD)) {
 		pk = get_xsave_addr(&new_fpu->state.xsave, XFEATURE_PKRU);
 		if (pk)
 			pkru_val = pk->pkru;

