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Date:   Thu, 09 Jul 2020 13:54:33 -0700 (PDT)
Subject:     Re: [PATCH 1/5] lib: Add a generic version of devmem_is_allowed()
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From:   Palmer Dabbelt <palmer@dabbelt.com>
To:     rppt@linux.ibm.com
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On Thu, 09 Jul 2020 13:49:21 PDT (-0700), rppt@linux.ibm.com wrote:
> Hi Palmer,
>
> On Thu, Jul 09, 2020 at 01:05:48PM -0700, Palmer Dabbelt wrote:
>> From: Palmer Dabbelt <palmerdabbelt@google.com>
>>
>> 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@google.com>
>> ---
>>  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.

Ya, it seemed kind of odd.

>
> 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.

There's some arches that can't be converted to the generic version, at least
not trivially, so we wouldn't drop it.  I think it's still cleaner, though.
I'll send a v2.

>
>> 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
>>