[PATCH v2 1/2] dma-direct: check for overflows on 32 bit DMA addresses

[Nicolas Saenz Julienne]

As seen on the new Raspberry Pi 4 and sta2x11's DMA implementation it is
possible for a device configured with 32 bit DMA addresses and a partial
DMA mapping located at the end of the address space to overflow. It
happens when a higher physical address, not DMAable, is translated to
it's DMA counterpart.

For example the Raspberry Pi 4, configurable up to 4 GB of memory, has
an interconnect capable of addressing the lower 1 GB of physical memory
with a DMA offset of 0xc0000000. It transpires that, any attempt to
translate physical addresses higher than the first GB will result in an
overflow which dma_capable() can't detect as it only checks for
addresses bigger then the maximum allowed DMA address.

Fix this by verifying in dma_capable() if the DMA address range provided
is at any point lower than the minimum possible DMA address on the bus.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@xxxxxxx>
---
 include/linux/dma-direct.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index adf993a3bd58..af4bb9c81ccc 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -3,6 +3,7 @@
 #define _LINUX_DMA_DIRECT_H 1
 
 #include <linux/dma-mapping.h>
+#include <linux/memblock.h> /* for min_low_pfn */
 #include <linux/mem_encrypt.h>
 
 #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA
@@ -27,6 +28,13 @@ static inline bool dma_capable(struct device *dev, dma_addr_t addr, size_t size)
 	if (!dev->dma_mask)
 		return false;
 
+#ifndef CONFIG_ARCH_DMA_ADDR_T_64BIT
+	/* Check if DMA address overflowed */
+	if (min(addr, addr + size - 1) <
+		__phys_to_dma(dev, (phys_addr_t)(min_low_pfn << PAGE_SHIFT)))
+		return false;
+#endif
+
 	return addr + size - 1 <=
 		min_not_zero(*dev->dma_mask, dev->bus_dma_mask);
 }
-- 
2.23.0