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[v4.4,02/45] arm64: Implement array_index_mask_nospec()

Message ID 3f1bc5e6cfd1d72a2e3612aca52a3811f38332cf.1560480942.git.viresh.kumar@linaro.org
State Superseded
Headers show
Series V4.4 backport of arm64 Spectre patches | expand

Commit Message

Viresh Kumar June 14, 2019, 3:07 a.m. UTC 
From: Robin Murphy <robin.murphy@arm.com>


commit 022620eed3d0bc4bf2027326f599f5ad71c2ea3f upstream.

Provide an optimised, assembly implementation of array_index_mask_nospec()
for arm64 so that the compiler is not in a position to transform the code
in ways which affect its ability to inhibit speculation (e.g. by introducing
conditional branches).

This is similar to the sequence used by x86, modulo architectural differences
in the carry/borrow flags.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Signed-off-by: Robin Murphy <robin.murphy@arm.com>

Signed-off-by: Will Deacon <will.deacon@arm.com>

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

---
 arch/arm64/include/asm/barrier.h | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

-- 
2.21.0.rc0.269.g1a574e7a288b
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Patch

diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 574486634c62..7c25e3e11b6d 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -37,6 +37,27 @@ 
 #define dma_rmb()	dmb(oshld)
 #define dma_wmb()	dmb(oshst)
 
+/*
+ * Generate a mask for array_index__nospec() that is ~0UL when 0 <= idx < sz
+ * and 0 otherwise.
+ */
+#define array_index_mask_nospec array_index_mask_nospec
+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, xzr, xzr\n"
+	: "=r" (mask)
+	: "r" (idx), "Ir" (sz)
+	: "cc");
+
+	csdb();
+	return mask;
+}
+
 #define smp_mb()	dmb(ish)
 #define smp_rmb()	dmb(ishld)
 #define smp_wmb()	dmb(ishst)