Re: [PATCH v2 2/2] kallsyms: handle special absolute symbols

[Kees Cook]

On Fri, Mar 7, 2014 at 5:00 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> This forces the entire per_cpu range to be reported as absolute
> without losing their linker symbol types, when the per_cpu area is
> 0-based. Without this, the variables are incorrectly shown as relocated
> under kASLR on x86_64.
>
> Several kallsyms output in different boot states for comparison of
> various symbols:
>
> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.nokaslr
> 0000000000000000 D __per_cpu_start
> 0000000000004000 D gdt_page
> 0000000000014280 D __per_cpu_end
> ffffffff810001c8 T _stext
> ffffffff81ee53c0 D __per_cpu_offset
> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr1
> 000000001f200000 D __per_cpu_start
> 000000001f204000 D gdt_page
> 000000001f214280 D __per_cpu_end
> ffffffffa02001c8 T _stext
> ffffffffa10e53c0 D __per_cpu_offset
> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr2
> 000000000d400000 D __per_cpu_start
> 000000000d404000 D gdt_page
> 000000000d414280 D __per_cpu_end
> ffffffff8e4001c8 T _stext
> ffffffff8f2e53c0 D __per_cpu_offset
> $ egrep ' (gdt_|_(stext|_per_cpu_))' /root/kallsyms.kaslr-fixed
> 0000000000000000 D __per_cpu_start
> 0000000000004000 D gdt_page
> 0000000000014280 D __per_cpu_end
> ffffffffadc001c8 T _stext
> ffffffffaeae53c0 D __per_cpu_offset
>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> v2:
>  - only force absolute when per_cpu starts at 0.
> ---
>  scripts/kallsyms.c |   20 +++++++++++++++++++-
>  1 file changed, 19 insertions(+), 1 deletion(-)
>
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index 08f30ac5b07d..d3f93b8eb277 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -34,6 +34,7 @@ struct sym_entry {
>         unsigned int len;
>         unsigned int start_pos;
>         unsigned char *sym;
> +       int force_absolute;
>  };
>
>  struct addr_range {
> @@ -51,6 +52,14 @@ static struct addr_range text_ranges[] = {
>  #define text_range_text     (&text_ranges[0])
>  #define text_range_inittext (&text_ranges[1])
>
> +/*
> + * Variables in these ranges, when the start is 0 based, will be forced to
> + * be handled as absolute addresses.
> + */
> +static struct addr_range abs_ranges[] = {
> +       { "__per_cpu_start",    "__per_cpu_end", -1ULL, 0 },
> +};
> +
>  static struct sym_entry *table;
>  static unsigned int table_size, table_cnt;
>  static int all_symbols = 0;
> @@ -165,6 +174,10 @@ static int read_symbol(FILE *in, struct sym_entry *s)
>         }
>         strcpy((char *)s->sym + 1, str);
>         s->sym[0] = stype;
> +       s->force_absolute = 0;
> +
> +       /* Check if we've found special absolute symbol range. */
> +       check_symbol_range(sym, s->addr, abs_ranges, ARRAY_SIZE(abs_ranges));
>
>         return 0;
>  }
> @@ -211,6 +224,11 @@ static int symbol_valid(struct sym_entry *s)
>         if (s->addr < kernel_start_addr)
>                 return 0;
>
> +       /* Force zero-based range special symbols into being absolute. */
> +       i = symbol_in_range(s, abs_ranges, ARRAY_SIZE(abs_ranges));
> +       if (i >= 0 && abs_ranges[i].start == 0)
> +               s->force_absolute = 1;

Rusty, is this 0-detection workable for you? If so, should you or akpm
carry this series for 3.15?

Thanks!

-Kees

> +
>         /* skip prefix char */
>         if (symbol_prefix_char && *(s->sym + 1) == symbol_prefix_char)
>                 offset++;
> @@ -307,7 +325,7 @@ static int expand_symbol(unsigned char *data, int len, char *result)
>
>  static int symbol_absolute(struct sym_entry *s)
>  {
> -       return toupper(s->sym[0]) == 'A';
> +       return s->force_absolute || toupper(s->sym[0]) == 'A';
>  }
>
>  static void write_src(void)
> --
> 1.7.9.5
>



-- 
Kees Cook
Chrome OS Security
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