[kernel-hardening] Re: [PATCH 2/3] vsprintf: print <no-symbol> if symbol not found

Joe Perches
2017-12-18T00:04:14Z

On Mon, 2017-12-18 at 10:53 +1100, Tobin C. Harding wrote:
> Depends on: commit bd6b239cdbb2 ("kallsyms: don't leak address when
> symbol not found")
>
> Currently vsprintf for specifiers %p[SsB] relies on the behaviour of
> kallsyms (sprint_symbol()) and prints the actual address if a symbol is
> not found. Previous patch changes this behaviour so tha sprint_symbol()

tha->that

> returns an error if symbol not found. With this patch in place we can
> print a sanitized message '<no-symbol>' instead of leaking the address.
>
> Print '<no-symbol>' for printk specifier %s[sSB] if no symbol is found.

%s->%ps

> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
[]
> @@ -460,6 +460,8 @@ char *kvasprintf(gfp_t gfp, const char *fmt, va_list args);
> extern __printf(2, 0)
> const char *kvasprintf_const(gfp_t gfp, const char *fmt, va_list args);
>
> +extern int string_is_no_symbol(const char *s);
> +
[]
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
>
> +#define PRINTK_NO_SYMBOL_STR "<no-symbol>"

"<symbol unavailable>" ? "not found"?

[]

> +int string_is_no_symbol(const char *s)
> +{
> + return !!strstr(s, PRINTK_NO_SYMBOL_STR);
> +}
> +EXPORT_SYMBOL(string_is_no_symbol);

Why should string_is_no_symbol be exported?