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

Tobin C. Harding
2017-12-19T06:33:06Z

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.