linux-kernel - Re: [PATCH 2/2] riscv: Support CONFIG_STRICT

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Date:   Wed, 17 Jun 2020 14:32:51 +0800
From:   Zong Li <zong.li@...ive.com>
To:     Nick Kossifidis <mick@....forth.gr>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] riscv: Support CONFIG_STRICT_DEVMEM

On Wed, Jun 17, 2020 at 1:28 PM Nick Kossifidis <mick@....forth.gr> wrote:
>
> Στις 2020-06-17 04:56, Zong Li έγραψε:
> > On Tue, Jun 16, 2020 at 8:27 PM Nick Kossifidis <mick@....forth.gr>
> > wrote:
> >>
> >> Στις 2020-06-16 10:45, Zong Li έγραψε:
> >> > 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
> >> >
> >> > Signed-off-by: Zong Li <zong.li@...ive.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;
> >>
> >> This shouldn't be part of /mm/init.c, it has nothing to do with memory
> >> initialization, I suggest we move it to another file like mmap.c on
> >
> > Let me move it, thanks.
> >
> >> arm/arm64. Also before using iomem_is_exclusive we should probably
> >> also
> >> mark any reserved regions with the no-map attribute as busy|exclusive,
> >> reserved-memory regions are not necessarily part of the main memory so
> >> the page_is_ram check may pass and iomem_is_exclusive won't do any
> >> good.
> >
> > What do you think if we mark the reserved region in
> > kdump_resource_init, and change the kdump_resource_init to a more
> > generic name for initializing resources?
>
> Sounds good to me, I'll work on this within the week. Do you agree with
> marking the no-map reserved-memory regions as exclusive ?

It's OK to me, It seems to me that exclusive is more suitable than
busy when initialization.