Re: [PATCH RFC 6/8] riscv/kaslr: clear the original kernel image

[Alex Ghiti]

On 3/24/20 3:30 AM, Zong Li wrote:
> After completing final page table, we can clear original kernel image
> and remove executable permission.
>
> Signed-off-by: Zong Li <zong.li@xxxxxxxxxx>
> ---
>   arch/riscv/include/asm/kaslr.h | 12 ++++++++++++
>   arch/riscv/kernel/kaslr.c      | 12 ++++++++++++
>   arch/riscv/mm/init.c           |  6 ++++++
>   3 files changed, 30 insertions(+)
>   create mode 100644 arch/riscv/include/asm/kaslr.h
>
> diff --git a/arch/riscv/include/asm/kaslr.h b/arch/riscv/include/asm/kaslr.h
> new file mode 100644
> index 000000000000..b165fe71dd4a
> --- /dev/null
> +++ b/arch/riscv/include/asm/kaslr.h
> @@ -0,0 +1,12 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2020 SiFive
> + * Copyright (C) 2020 Zong Li <zong.li@xxxxxxxxxx>
> + */
> +
> +#ifndef _ASM_RISCV_KASLR_H
> +#define _ASM_RISCV_KASLR_H
> +
> +void __init kaslr_late_init(void);
> +
> +#endif /* _ASM_RISCV_KASLR_H */
> diff --git a/arch/riscv/kernel/kaslr.c b/arch/riscv/kernel/kaslr.c
> index 59001d6fdfc3..0bd30831c455 100644
> --- a/arch/riscv/kernel/kaslr.c
> +++ b/arch/riscv/kernel/kaslr.c
> @@ -356,6 +356,18 @@ static __init uintptr_t get_random_offset(u64 seed, uintptr_t kernel_size)
>   	return get_legal_offset(random, kernel_size_align);
>   }
>   
> +void __init kaslr_late_init(void)
> +{
> +	uintptr_t kernel_size;
> +
> +	/* Clear original kernel image. */
> +	if (kaslr_offset) {
> +		kernel_size = (uintptr_t) _end - (uintptr_t) _start;

kernel_size = (uintptr_t) _end - (uintptr_t) _start + 1;

> +		memset((void *)PAGE_OFFSET, 0, kernel_size);

I have been thinking again about our discussion regarding PAGE_OFFSET: 
PAGE_OFFSET actually points to the address where the kernel was loaded, 
not the beginning of memory, that's a bit weird.

Just saying that here, because it took me a few seconds to remember that 
and understand what you were doing here.

> +		set_memory_nx(PAGE_OFFSET, kaslr_offset >> PAGE_SHIFT);

Again, I certainly missed something but when do you use old kernel 
mappings ?

> +	}
> +}
> +
>   uintptr_t __init kaslr_early_init(void)
>   {
>   	u64 seed;
> diff --git a/arch/riscv/mm/init.c b/arch/riscv/mm/init.c
> index 34c6ecf2c599..08e2ce170533 100644
> --- a/arch/riscv/mm/init.c
> +++ b/arch/riscv/mm/init.c
> @@ -15,6 +15,7 @@
>   #include <linux/set_memory.h>
>   #ifdef CONFIG_RELOCATABLE
>   #include <linux/elf.h>
> +#include <asm/kaslr.h>
>   #endif
>   
>   #include <asm/fixmap.h>
> @@ -649,6 +650,11 @@ static void __init setup_vm_final(void)
>   	/* Move to swapper page table */
>   	csr_write(CSR_SATP, PFN_DOWN(__pa_symbol(swapper_pg_dir)) | SATP_MODE);
>   	local_flush_tlb_all();
> +
> +#ifdef CONFIG_RANDOMIZE_BASE
> +	/* Clear orignial kernel image and set the right permission. */
> +	kaslr_late_init();
> +#endif
>   }
>   
>   void free_initmem(void)

Alex