[PATCH 2/2] riscv: Support CONFIG_STRICT_DEVMEM

[Palmer Dabbelt]

On Tue, 16 Jun 2020 00:45:47 PDT (-0700), zong.li at sifive.com wrote:
> 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.

In fact, it's exactly the same (down to a few words of the comment) to the
others that I checked.  I'm going to put a generic version in lib/ instead,
I've sent out the patches.

> 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 at sifive.com>
> ---
>  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;