x86/fpu: Clean up the fpu__clear() variants

fpu__clear() currently resets both register state and kernel XSAVE buffer
state.  It has two modes: one for all state (supervisor and user) and
another for user state only.  fpu__clear_all() uses the "all state"
(user_only=0) mode, while a number of signal paths use the user_only=1
mode.

Make fpu__clear() work only for user state (user_only=1) and remove the
"all state" (user_only=0) code.  Rename it to match so it can be used by
the signal paths.

Replace the "all state" (user_only=0) fpu__clear() functionality.  Use the
TIF_NEED_FPU_LOAD functionality instead of making any actual hardware
registers changes in this path.

Instead of invoking fpu__initialize() just memcpy() init_fpstate into the
task's FPU state because that has already the correct format and in case of
PKRU also contains the default PKRU value. Move the actual PKRU write out
into flush_thread() where it belongs and where it will end up anyway when
PKRU and XSTATE have been untangled.

For bisectability a workaround is required which stores the PKRU value in
the xstate memory until PKRU is untangled from XSTATE for context
switching and return to user.

[ Dave Hansen: Polished changelog ]
[ tglx: Fixed the PKRU fallout ]

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121455.922729522@linutronix.de

1 files changed, 10 insertions, 0 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 19d05d39a81d..de942b038815 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -198,6 +198,15 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg,
 	return ret;
 }
 
+static void pkru_flush_thread(void)
+{
+	/*
+	 * If PKRU is enabled the default PKRU value has to be loaded into
+	 * the hardware right here (similar to context switch).
+	 */
+	pkru_write_default();
+}
+
 void flush_thread(void)
 {
 	struct task_struct *tsk = current;
@@ -206,6 +215,7 @@ void flush_thread(void)
 	memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
 
 	fpu_flush_thread();
+	pkru_flush_thread();
 }
 
 void disable_TSC(void)