Re: [PATCH] kconfig: fix potential NULL pointer dereference in conf_askvalue

[Xingjing Deng]

Nathan Chancellor <nathan@xxxxxxxxxx> 于2026年2月26日周四 03:44写道：
>
> On Wed, Feb 25, 2026 at 07:22:46AM +0000, Xingjing Deng wrote:
> > In conf_askvalue(), the 'def' argument (retrieved via sym_get_string_value)
> > can be NULL. When the symbol is not changeable, the code calls
> > printf("%s\n", def), which leads to a segmentation fault on certain
> > systems/libc implementations when passing a NULL pointer to %s.
>
> How do you reproduce this segmentation fault? Surely someone would have
> hit this if it were a real problem given the Fixes tag? Or is this a
> corner case?

I tested printing NULL with printf locally and confirmed that it does
cause issues. In my opinion, this problem is more of a corner case—I
identified it through static program analysis and have not yet
reproduced it in practice.

>
> > This patch adds a check to ensure 'def' is not NULL before printing.
> > Additionally, it removes the redundant re-initialization of the 'line'
> > buffer inside the !sym_is_changeable(sym) block, as it is already
> > initialized at the beginning of the function.
> >
> > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > Signed-off-by: Xingjing Deng <micro6947@xxxxxxxxx>
> > ---
> >  scripts/kconfig/conf.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/scripts/kconfig/conf.c b/scripts/kconfig/conf.c
> > index a7b44cd8a..2771bc84e 100644
> > --- a/scripts/kconfig/conf.c
> > +++ b/scripts/kconfig/conf.c
> > @@ -297,9 +297,7 @@ static int conf_askvalue(struct symbol *sym, const char *def)
> >       line[1] = 0;
> >
> >       if (!sym_is_changeable(sym)) {
> > -             printf("%s\n", def);
> > -             line[0] = '\n';
> > -             line[1] = 0;
> > +             printf("%s\n", def ? def : "");
> >               return 0;
> >       }
> >
> > @@ -307,7 +305,7 @@ static int conf_askvalue(struct symbol *sym, const char *def)
> >       case oldconfig:
> >       case syncconfig:
> >               if (sym_has_value(sym)) {
> > -                     printf("%s\n", def);
> > +                     printf("%s\n", def ? def : "");
> >                       return 0;
> >               }
> >               /* fall through */
> > --
> > 2.25.1
> >