Re: [PATCH 1/5] lib: Add a generic version of devmem_is_allowed()

[Mike Rapoport]

Hi Palmer,

On Thu, Jul 09, 2020 at 01:05:48PM -0700, Palmer Dabbelt wrote:
> From: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> 
> As part of adding support for STRICT_DEVMEM to the RISC-V port, Zong
> provided a devmem_is_allowed() implementation that's exactly the same as
> all the others I checked.  Instead I'm adding a generic version, which
> will soon be used.
> 
> Signed-off-by: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> ---
>  include/asm-generic/io.h |  4 ++++
>  lib/Kconfig              |  4 ++++
>  lib/Makefile             |  2 ++
>  lib/devmem_is_allowed.c  | 27 +++++++++++++++++++++++++++
>  4 files changed, 37 insertions(+)
>  create mode 100644 lib/devmem_is_allowed.c
> 
> diff --git a/include/asm-generic/io.h b/include/asm-generic/io.h
> index 8b1e020e9a03..69e3db65fba0 100644
> --- a/include/asm-generic/io.h
> +++ b/include/asm-generic/io.h
> @@ -1122,6 +1122,10 @@ static inline void memcpy_toio(volatile void __iomem *addr, const void *buffer,
>  }
>  #endif
>  
> +#ifndef CONFIG_GENERIC_DEVMEM_IS_ALLOWED
> +extern int devmem_is_allowed(unsigned long pfn);
> +#endif
> +
>  #endif /* __KERNEL__ */
>  
>  #endif /* __ASM_GENERIC_IO_H */
> diff --git a/lib/Kconfig b/lib/Kconfig
> index df3f3da95990..3b1b6481e073 100644
> --- a/lib/Kconfig
> +++ b/lib/Kconfig
> @@ -676,3 +676,7 @@ config GENERIC_LIB_CMPDI2
>  
>  config GENERIC_LIB_UCMPDI2
>  	bool
> +
> +config GENERIC_LIB_DEVMEM_IS_ALLOWED
> +	bool
> +	select ARCH_HAS_DEVMEM_IS_ALLOWED

This seems to work the other way around from the usual Kconfig chains.
In the most cases ARCH_HAS_SOMETHING selects GENERIC_SOMETHING.

I believe nicer way would be to make 

config STRICT_DEVMEM
	bool "Filter access to /dev/mem"
	depends on MMU && DEVMEM
	depends on ARCH_HAS_DEVMEM_IS_ALLOWED || GENERIC_LIB_DEVMEM_IS_ALLOWED

config GENERIC_LIB_DEVMEM_IS_ALLOWED
	bool

and then s/select ARCH_HAS_DEVMEM_IS_ALLOWED/select GENERIC_LIB_DEVMEM_IS_ALLOWED/
in the arch Kconfigs and drop ARCH_HAS_DEVMEM_IS_ALLOWED in the end.

> diff --git a/lib/Makefile b/lib/Makefile
> index b1c42c10073b..554ef14f9be5 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -318,3 +318,5 @@ obj-$(CONFIG_OBJAGG) += objagg.o
>  # KUnit tests
>  obj-$(CONFIG_LIST_KUNIT_TEST) += list-test.o
>  obj-$(CONFIG_LINEAR_RANGES_TEST) += test_linear_ranges.o
> +
> +obj-$(CONFIG_GENERIC_LIB_DEVMEM_IS_ALLOWED) += devmem_is_allowed.o
> diff --git a/lib/devmem_is_allowed.c b/lib/devmem_is_allowed.c
> new file mode 100644
> index 000000000000..c0d67c541849
> --- /dev/null
> +++ b/lib/devmem_is_allowed.c
> @@ -0,0 +1,27 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * A generic version of devmem_is_allowed.
> + *
> + * Based on arch/arm64/mm/mmap.c
> + *
> + * Copyright (C) 2020 Google, Inc.
> + * Copyright (C) 2012 ARM Ltd.
> + */
> +
> +#include <linux/mm.h>
> +#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.  We mimic x86 here by
> + * disallowing 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;
> +}
> -- 
> 2.27.0.383.g050319c2ae-goog
> 

-- 
Sincerely yours,
Mike.