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Subject: Re: [PATCH 2/3] vsprintf: print <no-symbol> if symbol not found
From: Joe Perches <joe@perches.com>
To: "Tobin C. Harding" <me@tobin.cc>, kernel-hardening@lists.openwall.com
Cc: Steven Rostedt <rostedt@goodmis.org>, Tycho Andersen <tycho@tycho.ws>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        Alexei Starovoitov <ast@kernel.org>, linux-kernel@vger.kernel.org,
        Network Development <netdev@vger.kernel.org>
Date: Sun, 17 Dec 2017 16:04:14 -0800
In-Reply-To: <1513554812-13014-3-git-send-email-me@tobin.cc>
References: <1513554812-13014-1-git-send-email-me@tobin.cc>
         <1513554812-13014-3-git-send-email-me@tobin.cc>
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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?