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    SubjectRe: [PATCH 2/2] riscv: Support CONFIG_STRICT_DEVMEM
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    On Tue, 16 Jun 2020 00:45:47 PDT (-0700), zong.li@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@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;

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