FAILED: patch "[PATCH] arm64: fpsimd: Prevent registers leaking across exec" failed to apply to 4.9-stable tree

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From: Dave Martin <[email protected]>
Date: Fri, 18 Aug 2017 16:57:01 +0100
Subject: [PATCH] arm64: fpsimd: Prevent registers leaking across exec

There are some tricky dependencies between the different stages of flushing the FPSIMD register state during exec, and these can race with context switch in ways that can cause the old task's regs to leak across. In particular, a context switch during the memset() can cause some of the task's old FPSIMD registers to reappear.

Disabling preemption for this small window would be no big deal for
performance: preemption is already disabled for similar scenarios like updating the FPSIMD registers in sigreturn.

So, instead of rearranging things in ways that might swap existing subtle bugs for new ones, this patch just disables preemption around the FPSIMD state flushing so that races of this type can't occur here. This brings fpsimd_flush_thread() into line with other code paths.

index 06da8ea16bbe..c7b4995868e1 100644
@@ -161,9 +161,11 @@ void fpsimd_flush_thread(void)
 {
 	if (!system_supports_fpsimd())
 		return;
+	preempt_disable();
 	memset(&current->thread.fpsimd_state, 0, sizeof(struct fpsimd_state));
 	fpsimd_flush_task_state(current);
 	set_thread_flag(TIF_FOREIGN_FPSTATE);
+	preempt_enable();
 }
 
 /*