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

[Zong Li <zong.li@sifive.com>]

On Tue, Apr 7, 2020 at 1:11 PM Alex Ghiti <alex@ghiti.fr> wrote:
>
> 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@sifive.com>
> > ---
> >   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@sifive.com>
> > + */
> > +
> > +#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;

OK, change it in the next version. Thanks.

>
> > +             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.

In non-kaslr case, we load the kernel to PAGE_OFFSET which points to,
so we clear the old kernel image through PAGE_OFFSET here. Certainly,
we could use a symbol to record the start address of the old kernel
image instead of PAGE_OFFSET here. I don't see other architectures
changing PAGE_OFFSET after copying the kernel to the new location in
kaslr. If you think the PAGE_OFFSET needs to be changed, we need to
give another way to make the page table could create the mappings for
the whole memory and memblock/buddy system could see the whole memory
after the kernel moves.

>
> > +             set_memory_nx(PAGE_OFFSET, kaslr_offset >> PAGE_SHIFT);
>
> Again, I certainly missed something but when do you use old kernel
> mappings ?

We use old kernel mappings when KASLR calculates the random offset, at
that moment, kernel is running on old kernel location.

>
> > +     }
> > +}
> > +
> >   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