[kernel-hardening] Re: [PATCH v2 2/3] vsprintf: print <no-symbol> if symbol not found

[Joe Perches <joe@perches.com>]

On Tue, 2017-12-19 at 14:28 +1100, Tobin C. Harding wrote:
> Depends on: commit 40eee173a35e ("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 that sprint_symbol()
> returns an error if symbol not found. With this patch in place we can
> print a sanitized message '<symbol not found>' instead of leaking the
> address.
> 
> Print '<symbol not found>' for printk specifier %p[sSB] if symbol look
> up fails.
> 
> Signed-off-by: Tobin C. Harding <me@tobin.cc>
> ---
>  lib/vsprintf.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 01c3957b2de6..820ed4fe6e6c 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -674,6 +674,8 @@ char *symbol_string(char *buf, char *end, void *ptr,
>  	unsigned long value;
>  #ifdef CONFIG_KALLSYMS
>  	char sym[KSYM_SYMBOL_LEN];
> +	const char *sym_not_found = "<symbol not found>";

This will be reinitialized on every use.

> +	int ret;
>  #endif
>  
>  	if (fmt[1] == 'R')
> @@ -682,11 +684,14 @@ char *symbol_string(char *buf, char *end, void *ptr,
>  
>  #ifdef CONFIG_KALLSYMS
>  	if (*fmt == 'B')
> -		sprint_backtrace(sym, value);
> +		ret = sprint_backtrace(sym, value);
>  	else if (*fmt != 'f' && *fmt != 's')
> -		sprint_symbol(sym, value);
> +		ret = sprint_symbol(sym, value);
>  	else
> -		sprint_symbol_no_offset(sym, value);
> +		ret = sprint_symbol_no_offset(sym, value);
> +
> +	if (ret == -1)
> +		strcpy(sym, sym_not_found);


This could avoid the unnecessary strcpy if sym_not_found
was not used at all and this was used instead

	if (ret == -1)
		return string(buf, end, "<symbol not found>", spec);

	return string(buf, end, sym, spec);

or maybe

	return string(buf, end, ret == -1 ? "<symbol not found>" : sum, spec);

>  
>  	return string(buf, end, sym, spec);
>  #else