linux-kernel - [PATCH 5.4 66/90] x86/process: Check PF_KTHREAD and not current->mm for kernel threads

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Message-Id: <20210621154906.393789166@linuxfoundation.org>
Date:   Mon, 21 Jun 2021 18:15:41 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...e.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...riel.com>
Subject: [PATCH 5.4 66/90] x86/process: Check PF_KTHREAD and not current->mm for kernel threads

From: Thomas Gleixner <tglx@...utronix.de>

commit 12f7764ac61200e32c916f038bdc08f884b0b604 upstream.

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@...utronix.de>
Signed-off-by: Borislav Petkov <bp@...e.de>
Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>
Acked-by: Rik van Riel <riel@...riel.com>
Cc: stable@...r.kernel.org
Link: https://lkml.kernel.org/r/20210608144345.912645927@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.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
@@ -607,7 +607,7 @@ 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;