linux-kernel - Re: [PATCH v3 34/60] arm64: head: Move early kernel mapping routines into C code

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Message-ID: <CAMj1kXGJSnoLj=eyQJnSthuuyRHSKTD1+RpyMu33ps3Sp6cb+Q@mail.gmail.com>
Date:   Tue, 18 Apr 2023 12:06:49 +0200
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v3 34/60] arm64: head: Move early kernel mapping routines
 into C code

On Tue, 18 Apr 2023 at 11:31, Ryan Roberts <ryan.roberts@....com> wrote:
>
> On 07/03/2023 14:04, Ard Biesheuvel wrote:
> > The asm version of the kernel mapping code works fine for creating a
> > coarse grained identity map, but for mapping the kernel down to its
> > exact boundaries with the right attributes, it is not suitable. This is
> > why we create a preliminary RWX kernel mapping first, and then rebuild
> > it from scratch later on.
> >
> > So let's reimplement this in C, in a way that will make it unnecessary
> > to create the kernel page tables yet another time in paging_init().
> >
> > Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> > ---
>
> > [...]
>
> > diff --git a/arch/arm64/kernel/pi/map_kernel.c b/arch/arm64/kernel/pi/map_kernel.c
> > new file mode 100644
> > index 0000000000000000..b573c964c7d88d1b
> > --- /dev/null
> > +++ b/arch/arm64/kernel/pi/map_kernel.c
> > @@ -0,0 +1,171 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +// Copyright 2023 Google LLC
> > +// Author: Ard Biesheuvel <ardb@...gle.com>
> > +
> > +#include <linux/init.h>
> > +#include <linux/libfdt.h>
> > +#include <linux/linkage.h>
> > +#include <linux/types.h>
> > +#include <linux/sizes.h>
> > +#include <linux/string.h>
> > +
> > +#include <asm/memory.h>
> > +#include <asm/pgalloc.h>
> > +#include <asm/pgtable.h>
> > +#include <asm/tlbflush.h>
> > +
> > +#include "pi.h"
> > +
> > +extern const u8 __eh_frame_start[], __eh_frame_end[];
> > +
> > +extern void idmap_cpu_replace_ttbr1(void *pgdir);
> > +
> > +static void map_segment(pgd_t *pg_dir, u64 *pgd, u64 va_offset,
> > +                     void *start, void *end, pgprot_t prot,
> > +                     bool may_use_cont, int root_level)
> > +{
> > +     map_range(pgd, ((u64)start + va_offset) & ~PAGE_OFFSET,
> > +               ((u64)end + va_offset) & ~PAGE_OFFSET, (u64)start,
> > +               prot, root_level, (pte_t *)pg_dir, may_use_cont, 0);
>
> I don't understand what you are doing with ~PAGE_OFFSET here. Is this intended
> to be page alignment with PAGE_MASK? I'm guessing not, because you would want to
> forward align the end address in that case.
>

start + va_offset will produce an address that has leading 1 bits set
in positions that do not contribute to the translation. In order to
index the page tables correctly, those bits need to be cleared.

> > +}
> > +
> > +static void unmap_segment(pgd_t *pg_dir, u64 va_offset, void *start,
> > +                       void *end, int root_level)
> > +{
> > +     map_segment(pg_dir, NULL, va_offset, start, end, __pgprot(0),
> > +                 false, root_level);
> > +}
> > +
>
> > [...]
>
> > diff --git a/arch/arm64/kernel/pi/map_range.c b/arch/arm64/kernel/pi/map_range.c
> > new file mode 100644
> > index 0000000000000000..61cbd6e82418c033
> > --- /dev/null
> > +++ b/arch/arm64/kernel/pi/map_range.c
> > @@ -0,0 +1,87 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +// Copyright 2023 Google LLC
> > +// Author: Ard Biesheuvel <ardb@...gle.com>
> > +
> > +#include <linux/types.h>
> > +#include <linux/sizes.h>
> > +
> > +#include <asm/memory.h>
> > +#include <asm/pgalloc.h>
> > +#include <asm/pgtable.h>
> > +
> > +#include "pi.h"
> > +
> > +/**
> > + * map_range - Map a contiguous range of physical pages into virtual memory
> > + *
> > + * @pte:             Address of physical pointer to array of pages to
> > + *                   allocate page tables from
> > + * @start:           Virtual address of the start of the range
> > + * @end:             Virtual address of the end of the range (exclusive)
> > + * @pa:                      Physical address of the start of the range
> > + * @level:           Translation level for the mapping
> > + * @tbl:             The level @level page table to create the mappings in
> > + * @may_use_cont:    Whether the use of the contiguous attribute is allowed
> > + * @va_offset:               Offset between a physical page and its current mapping
> > + *                   in the VA space
> > + */
> > +void __init map_range(u64 *pte, u64 start, u64 end, u64 pa, pgprot_t prot,
> > +                   int level, pte_t *tbl, bool may_use_cont, u64 va_offset)
>
> va_offset is always 0 (because the memory at *pte is id-mapped). Can it be
> dropped? Or perhaps you are using this function later, once the memory is no
> longer id-mapped?
>

It will be used later.