[PATCH 2/2] arm64: mm: dump: don't skip final region

[Steve Capper]

On 5 December 2014 at 12:34, Mark Rutland <mark.rutland at arm.com> wrote:
> If the final page table entry we walk is a valid mapping, the page table
> dumping code will not log the region this entry is part of, as the final
> note_page call in ptdump_show will trigger an early return. Luckily this
> isn't seen on contemporary systems as they typically don't have enough
> RAM to extend the linear mapping right to the end of the address space.
>
> In note_page, we log a region  when we reach its end (i.e. we hit an
> entry immediately afterwards which has different prot bits or is
> invalid). The final entry has no subsequent entry, so we will not log
> this immediately. We try to cater for this with a subsequent call to
> note_page in ptdump_show, but this returns early as 0 < LOWEST_ADDR, and
> hence we will skip a valid mapping if it spans to the final entry we
> note.
>
> Unlike 32-bit ARM, the pgd with the kernel mapping is never shared with
> user mappings, so we do not need the check to ensure we don't log user
> page tables. Due to the way addr is constructed in the walk_* functions,
> it can never be less than LOWEST_ADDR when walking the page tables, so
> it is not necessary to avoid dereferencing invalid table addresses. The
> existing checks for st->current_prot and st->marker[1].start_address are
> sufficient to ensure we will not print and/or dereference garbage when
> trying to log information.
>
> This patch removes the unnecessary check against LOWEST_ADDR, ensuring
> we log all regions in the kernel page table, including those which span
> right to the end of the address space.
>

Acked-by: Steve Capper <steve.capper at linaro.org>

> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
> Cc: Kees Cook <keescook at chromium.org>
> Cc: Laura Abbott <lauraa at codeaurora.org>
> Cc: Steve Capper <steve.capper at linaro.org>
> Cc: Will Deacon <will.deacon at arm.com>
> ---
>  arch/arm64/mm/dump.c | 3 ---
>  1 file changed, 3 deletions(-)
>
> diff --git a/arch/arm64/mm/dump.c b/arch/arm64/mm/dump.c
> index a546776..cf33f33 100644
> --- a/arch/arm64/mm/dump.c
> +++ b/arch/arm64/mm/dump.c
> @@ -182,9 +182,6 @@ static void note_page(struct pg_state *st, unsigned long addr, unsigned level,
>         static const char units[] = "KMGTPE";
>         u64 prot = val & pg_level[level].mask;
>
> -       if (addr < LOWEST_ADDR)
> -               return;
> -
>         if (!st->level) {
>                 st->level = level;
>                 st->current_prot = prot;
> --
> 1.9.1
>