[tip: x86/urgent] x86/process: Check PF_KTHREAD and not current->mm for kernel threads

[tip-bot2 for Thomas Gleixner]

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

Commit-ID:     12f7764ac61200e32c916f038bdc08f884b0b604
Gitweb:        https://git.kernel.org/tip/12f7764ac61200e32c916f038bdc08f884b0b604
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 08 Jun 2021 16:36:20 +02:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 09 Jun 2021 10:39:04 +02:00

x86/process: Check PF_KTHREAD and not current->mm for kernel threads

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@xxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20210608144345.912645927@xxxxxxxxxxxxx
---
 arch/x86/include/asm/fpu/internal.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
index ceeba9f..18382ac 100644
--- 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(struct fpu *new_fpu)
 	 * 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;