[PATCH v2 1/2] 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

Changes in v2:
 - Move implementations out from mm/init.c. Suggested by Nick Kossifidis.

Signed-off-by: Zong Li <zong.li at sifive.com>
---
 arch/riscv/Kconfig          |  1 +
 arch/riscv/include/asm/io.h |  2 ++
 arch/riscv/mm/Makefile      |  1 +
 arch/riscv/mm/mmap.c        | 24 ++++++++++++++++++++++++
 4 files changed, 28 insertions(+)
 create mode 100644 arch/riscv/mm/mmap.c

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index 089293e4ad46..c6569939f7ba 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -17,6 +17,7 @@ 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/Makefile b/arch/riscv/mm/Makefile
index 363ef01c30b1..4530ad52f1d6 100644
--- a/arch/riscv/mm/Makefile
+++ b/arch/riscv/mm/Makefile
@@ -10,6 +10,7 @@ obj-y += extable.o
 obj-$(CONFIG_MMU) += fault.o pageattr.o
 obj-y += cacheflush.o
 obj-y += context.o
+obj-y += mmap.o
 
 ifeq ($(CONFIG_MMU),y)
 obj-$(CONFIG_SMP) += tlbflush.o
diff --git a/arch/riscv/mm/mmap.c b/arch/riscv/mm/mmap.c
new file mode 100644
index 000000000000..095fc82897b1
--- /dev/null
+++ b/arch/riscv/mm/mmap.c
@@ -0,0 +1,24 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2020 SiFive
+ */
+
+#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
-- 
2.27.0