ARM: spectre-v1: add array_index_mask_nospec() implementation

Add an implementation of the array_index_mask_nospec() function for
mitigating Spectre variant 1 throughout the kernel.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Boot-tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Tony Lindgren <tony@atomide.com>

1 files changed, 19 insertions, 0 deletions

diff --git a/arch/arm/include/asm/barrier.h b/arch/arm/include/asm/barrier.h
index 3d9c1d4b7e75..69772e742a0a 100644
--- a/arch/arm/include/asm/barrier.h
+++ b/arch/arm/include/asm/barrier.h
@@ -76,6 +76,25 @@ extern void arm_heavy_mb(void);
 #define __smp_rmb()     __smp_mb()
 #define __smp_wmb()     dmb(ishst)
 
+#ifdef CONFIG_CPU_SPECTRE
+static inline unsigned long array_index_mask_nospec(unsigned long idx,
+                                                    unsigned long sz)
+{
+        unsigned long mask;
+
+        asm volatile(
+                "cmp    %1, %2\n"
+        "       sbc     %0, %1, %1\n"
+        CSDB
+        : "=r" (mask)
+        : "r" (idx), "Ir" (sz)
+        : "cc");
+
+        return mask;
+}
+#define array_index_mask_nospec array_index_mask_nospec
+#endif
+
 #include <asm-generic/barrier.h>
 
 #endif /* !__ASSEMBLY__ */