linux-kernel - [PATCH v2 3/3] arm64: realm: Use aliased addresses for device DMA to shared buffers

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Message-ID: <20250219220751.1276854-4-suzuki.poulose@arm.com>
Date: Wed, 19 Feb 2025 22:07:51 +0000
From: Suzuki K Poulose <suzuki.poulose@....com>
To: will@...nel.org,
	robin.murphy@....com,
	catalin.marinas@....com
Cc: maz@...nel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org,
	gregkh@...uxfoundation.org,
	aneesh.kumar@...nel.org,
	steven.price@....com,
	suzuki.poulose@....com,
	Jean-Philippe Brucker <jean-philippe@...aro.org>,
	Christoph Hellwig <hch@....de>,
	Tom Lendacky <thomas.lendacky@....com>
Subject: [PATCH v2 3/3] arm64: realm: Use aliased addresses for device DMA to shared buffers

When a device performs DMA to a shared buffer using physical addresses,
(without Stage1 translation), the device must use the "{I}PA address" with the
top bit set in Realm. This is to make sure that a trusted device will be able
to write to shared buffers as well as the protected buffers. Thus, a Realm must
always program the full address including the "protection" bit, like AMD SME
encryption bits.

Enable this by providing arm64 specific dma_{encrypted,decrypted,clear_encryption}
helpers for Realms. Please note that the VMM needs to similarly make sure that
the SMMU Stage2 in the Non-secure world is setup accordingly to map IPA at the
unprotected alias.

Cc: Will Deacon <will@...nel.org>
Cc: Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc: Catalin Marinas <catalin.marinas@....com>
Cc: Robin Murphy <robin.murphy@....com>
Cc: Steven Price <steven.price@....com>
Cc: Christoph Hellwig <hch@....de>
Cc: Tom Lendacky <thomas.lendacky@....com>
Cc: Aneesh Kumar K.V <aneesh.kumar@...nel.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
---
 arch/arm64/include/asm/mem_encrypt.h | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/arm64/include/asm/mem_encrypt.h b/arch/arm64/include/asm/mem_encrypt.h
index f8f78f622dd2..aeda3bba255e 100644
--- a/arch/arm64/include/asm/mem_encrypt.h
+++ b/arch/arm64/include/asm/mem_encrypt.h
@@ -21,4 +21,26 @@ static inline bool force_dma_unencrypted(struct device *dev)
 	return is_realm_world();
 }
 
+static inline dma_addr_t dma_decrypted(dma_addr_t daddr)
+{
+	if (is_realm_world())
+		daddr |= prot_ns_shared;
+	return daddr;
+}
+#define dma_decrypted dma_decrypted
+
+static inline dma_addr_t dma_encrypted(dma_addr_t daddr)
+{
+	if (is_realm_world())
+		daddr &= prot_ns_shared - 1;
+	return daddr;
+}
+#define dma_encrypted dma_encrypted
+
+static inline dma_addr_t dma_clear_encryption(dma_addr_t daddr)
+{
+	return dma_encrypted(daddr);
+}
+#define dma_clear_encryption dma_clear_encryption
+
 #endif	/* __ASM_MEM_ENCRYPT_H */
-- 
2.43.0