[PATCH] arm64: fpsimd: Prevent registers leaking across exec

[Dave Martin <Dave.Martin@xxxxxxx>]

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.

Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Fixes: 674c242c9323 ("arm64: flush FP/SIMD state correctly after execve()")
Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
---

NOTE TO MAINTAINERS: This is a fix, applicable to *v4.13* and stable.

The kernel-mode NEON rework already queued for v4.14 turns this into a
local_bh_disable() as a side effect, so does not strictly need this
patch.  When merging, the queued v4.14 changes should be retained.

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

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 06da8ea..c7b4995 100644
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -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();
 }
 
 /*
-- 
2.1.4