Patch "arm64: fpsimd: Prevent registers leaking across exec" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

arm64: fpsimd: Prevent registers leaking across exec

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
arm64-fpsimd-prevent-registers-leaking-across-exec.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

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

From: Dave Martin <[email protected]>

commit 096622104e14d8a1db4860bd557717067a0515d2 upstream.

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.

---

For stable only.

3.17.x-4.0.x don't appear active, and this patch isn't sufficient to fix them (they would need 674c242c9323 also).

 arch/arm64/kernel/fpsimd.c |    2 ++
 1 file changed, 2 insertions(+)

@@ -157,9 +157,11 @@ void fpsimd_thread_switch(struct task_st
 
 void fpsimd_flush_thread(void)
 {
+	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();
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/arm64-fpsimd-prevent-registers-leaking-across-exec.patch