Re: [PATCH 2/2] ocfs2: Replace strlcpy with strscpy

[Azeem Shaikh <azeemshaikh38@gmail.com>]

On Wed, Aug 30, 2023 at 7:06 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Aug 30, 2023 at 09:54:26PM +0000, Azeem Shaikh wrote:
> > strlcpy() reads the entire source buffer first.
> > This read may exceed the destination size limit.
> > This is both inefficient and can lead to linear read
> > overflows if a source string is not NUL-terminated [1].
> > In an effort to remove strlcpy() completely [2], replace
> > strlcpy() here with strscpy().
> >
> > Direct replacement is assumed to be safe here since
> > it's ok for `kernel_param_ops.get()` to return -errno [3].
> > This changes the behavior such that instead of silently ignoring the
> > case when sizeof(@buffer) < DLMFS_CAPABILITIES, we now return error.
> >
> > [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strlcpy
> > [2] https://github.com/KSPP/linux/issues/89
> > [3] https://elixir.bootlin.com/linux/v6.5/source/include/linux/moduleparam.h#L52
> >
> > Signed-off-by: Azeem Shaikh <azeemshaikh38@gmail.com>
> > ---
> >  fs/ocfs2/dlmfs/dlmfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/ocfs2/dlmfs/dlmfs.c b/fs/ocfs2/dlmfs/dlmfs.c
> > index 33e529de93b2..b001eccdd2f3 100644
> > --- a/fs/ocfs2/dlmfs/dlmfs.c
> > +++ b/fs/ocfs2/dlmfs/dlmfs.c
> > @@ -80,7 +80,7 @@ static int param_set_dlmfs_capabilities(const char *val,
> >  static int param_get_dlmfs_capabilities(char *buffer,
> >                                       const struct kernel_param *kp)
> >  {
> > -     return strlcpy(buffer, DLMFS_CAPABILITIES,
> > +     return strscpy(buffer, DLMFS_CAPABILITIES,
> >                      strlen(DLMFS_CAPABILITIES) + 1);
> >  }
>
> This is another case of "accidentally correct".
>
>
> param->get() is hooked here, in the sysfs "show" callback:
>
> static ssize_t param_attr_show(struct module_attribute *mattr,
>                                struct module_kobject *mk, char *buf)
> {
>         int count;
>         struct param_attribute *attribute = to_param_attr(mattr);
>
>         if (!attribute->param->ops->get)
>                 return -EPERM;
>
>         kernel_param_lock(mk->mod);
>         count = attribute->param->ops->get(buf, attribute->param);
>         kernel_param_unlock(mk->mod);
>         return count;
> }
>
> Meaning ultimately this will show up here, if I'm reading names right:
> /sys/module/ocfs/parameters/dlmfs_capabilities
>
> Anyway, the "count" being returned would be quite bad if
> DLMFS_CAPABILITIES were dynamic and larger than PAGE_SIZE (the size of
> the sysfs buffer).
>
> For this case, I would say replace strlcpy with sysfs_emit:
>
>         return sysfs_emit(buffer, DLMFS_CAPABILITIES);
>

Thanks, sending out a v2 for this. Out of curiosity - why sysfs_emit?
Is it because DLMFS_CAPABILITIES is a hard-coded string?