arm64: neon: Forbid when irqs are disabled

Currently, may_use_simd() can return true if IRQs are disabled.  If
the caller goes ahead and calls kernel_neon_begin(), this can
result in use of local_bh_enable() in an unsafe context.

In particular, __efi_fpsimd_begin() may do this when calling EFI as
part of system shutdown.

This patch ensures that callers don't think they can use
kernel_neon_begin() in such a context.
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/simd.h

@@ -10,6 +10,7 @@
 #define __ASM_SIMD_H
 
 #include <linux/compiler.h>
+#include <linux/irqflags.h>
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/types.h>
@@ -40,7 +41,8 @@ static __must_check inline bool may_use_simd(void)
 	 * can't migrate to another CPU and spuriously see it become
 	 * false.
 	 */
-	return !in_irq() && !in_nmi() && !raw_cpu_read(kernel_neon_busy);
+	return !in_irq() && !irqs_disabled() && !in_nmi() &&
+		!raw_cpu_read(kernel_neon_busy);
 }
 
 #else /* ! CONFIG_KERNEL_MODE_NEON */