4.4-stable review patch. If anyone has any objections, please let me know. ------------------ From: Dave Martin <Dave.Martin@xxxxxxx> 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. Fixes: 674c242c9323 ("arm64: flush FP/SIMD state correctly after execve()") Reviewed-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx> Signed-off-by: Will Deacon <will.deacon@xxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- 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(+) --- a/arch/arm64/kernel/fpsimd.c +++ b/arch/arm64/kernel/fpsimd.c @@ -157,9 +157,11 @@ void fpsimd_thread_switch(struct task_st void fpsimd_flush_thread(void) { + preempt_disable(); memset(¤t->thread.fpsimd_state, 0, sizeof(struct fpsimd_state)); fpsimd_flush_task_state(current); set_thread_flag(TIF_FOREIGN_FPSTATE); + preempt_enable(); } /*