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

[Palmer Dabbelt]

On Thu, 09 Jul 2020 13:49:21 PDT (-0700), rppt@xxxxxxxxxxxxx wrote:
> 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.

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