[PATCH 2/2] riscv: Support CONFIG_STRICT_DEVMEM

[Zong Li]

Implement the 'devmem_is_allowed()' interface for RISC-V, like some of
other architectures have done. It will be called from range_is_allowed()
when userpsace attempts to access /dev/mem.

Access to exclusive IOMEM and kernel RAM is denied unless
CONFIG_STRICT_DEVMEM is set to 'n'.

Test it by devmem, the result as follows:

 - CONFIG_STRICT_DEVMEM=y
	$ devmem 0x10010000
	0x00000000
	$ devmem 0x80200000
	0x0000106F

 - CONFIG_STRICT_DEVMEM is not set
	$ devmem 0x10010000
	devmem: mmap: Operation not permitted
	$ devmem 0x80200000
	devmem: mmap: Operation not permitted

Signed-off-by: Zong Li <zong.li@xxxxxxxxxx>
---
 arch/riscv/Kconfig          |  1 +
 arch/riscv/include/asm/io.h |  2 ++
 arch/riscv/mm/init.c        | 19 +++++++++++++++++++
 3 files changed, 22 insertions(+)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 128192e14ff2..ffd7841ede4c 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -16,6 +16,7 @@ config RISCV
 	select ARCH_HAS_BINFMT_FLAT
 	select ARCH_HAS_DEBUG_VIRTUAL if MMU
 	select ARCH_HAS_DEBUG_WX
+	select ARCH_HAS_DEVMEM_IS_ALLOWED
 	select ARCH_HAS_GCOV_PROFILE_ALL
 	select ARCH_HAS_GIGANTIC_PAGE
 	select ARCH_HAS_MMIOWB
diff --git a/arch/riscv/include/asm/io.h b/arch/riscv/include/asm/io.h
index 3835c3295dc5..04ac65ab93ce 100644
--- a/arch/riscv/include/asm/io.h
+++ b/arch/riscv/include/asm/io.h
@@ -147,4 +147,6 @@ __io_writes_outs(outs, u64, q, __io_pbr(), __io_paw())
 
 #include <asm-generic/io.h>
 
+extern int devmem_is_allowed(unsigned long pfn);
+
 #endif /* _ASM_RISCV_IO_H */
diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
index bbe816e03b2f..5e7e61519acc 100644
--- a/arch/riscv/mm/init.c
+++ b/arch/riscv/mm/init.c
@@ -517,6 +517,25 @@ void mark_rodata_ro(void)
 }
 #endif
 
+#ifdef CONFIG_STRICT_DEVMEM
+#include <linux/ioport.h>
+/*
+ * devmem_is_allowed() checks to see if /dev/mem access to a certain address
+ * is valid. The argument is a physical page number.
+ *
+ * Disallow access to system RAM as well as device-exclusive MMIO regions.
+ * This effectively disable read()/write() on /dev/mem.
+ */
+int devmem_is_allowed(unsigned long pfn)
+{
+	if (iomem_is_exclusive(pfn << PAGE_SHIFT))
+		return 0;
+	if (!page_is_ram(pfn))
+		return 1;
+	return 0;
+}
+#endif
+
 void __init resource_init(void)
 {
 	struct memblock_region *region;
-- 
2.27.0