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

["Tobin C. Harding" <me@tobin.cc>]

On Mon, Dec 18, 2017 at 10:18:27PM -0800, Joe Perches wrote:
> 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);

Oh, thanks. This is much cleaner. Will re-spin.

thanks,
Tobin.