[PATCH RFC 4/5] dma/direct: check for overflows in ARM's dma_capable()

[Nicolas Saenz Julienne]

The Raspberry Pi 4 has a 1GB ZONE_DMA area starting at address
0x00000000 and a mapping between physical and DMA memory offset by
0xc0000000.  It transpires that, on non LPAE systems, any attempt to
translate physical addresses outside of ZONE_DMA will result in an
overflow. The resulting DMA addresses will not be detected by arm's
dma_capable() as they still fit in the device's DMA mask.

Fix this by failing to validate a DMA address smaller than the lowest
possible DMA address.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
---
 arch/arm/include/asm/dma-direct.h | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/arm/include/asm/dma-direct.h b/arch/arm/include/asm/dma-direct.h
index b67e5fc1fe43..ee8ad47a14e3 100644
--- a/arch/arm/include/asm/dma-direct.h
+++ b/arch/arm/include/asm/dma-direct.h
@@ -2,6 +2,8 @@
 #ifndef ASM_ARM_DMA_DIRECT_H
 #define ASM_ARM_DMA_DIRECT_H 1
 
+#include <linux/memblock.h>
+
 static inline dma_addr_t __phys_to_dma(struct device *dev, phys_addr_t paddr)
 {
 	unsigned int offset = paddr & ~PAGE_MASK;
@@ -21,6 +23,10 @@ static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
 	if (!dev->dma_mask)
 		return 0;
 
+	/* Check if address overflowed */
+	if (addr < __phys_to_dma(dev, PFN_UP(min_low_pfn)))
+		return 0;
+
 	mask = *dev->dma_mask;
 
 	limit = (mask + 1) & ~mask;
-- 
2.23.0