Re: [PATCH v3 4/4] mm: huge_memory: Use strscpy() instead of strcpy()

[Barry Song <21cnbao@gmail.com>]

On Fri, Nov 1, 2024 at 9:27 AM Barry Song <21cnbao@gmail.com> wrote:
>
> On Fri, Nov 1, 2024 at 1:12 AM Maíra Canal <mcanal@igalia.com> wrote:
> >
> > +cc Kees Cook
> >
> > Hi Barry,
> >
> > On 31/10/24 09:01, Barry Song wrote:
> > > On Thu, Oct 31, 2024 at 6:55 PM Maíra Canal <mcanal@igalia.com> wrote:
> > >>
> > >> Hi Barry,
> > >>
> > >> On 30/10/24 20:07, Barry Song wrote:
> > >>> On Thu, Oct 31, 2024 at 2:03 AM Maíra Canal <mcanal@igalia.com> wrote:
> > >>>>
> > >>>> Replace strcpy() with strscpy() in mm/huge_memory.c
> > >>>>
> > >>>> strcpy() has been deprecated because it is generally unsafe, so help to
> > >>>> eliminate it from the kernel source.
> > >>>>
> > >>>> Link: https://github.com/KSPP/linux/issues/88
> > >>>> Signed-off-by: Maíra Canal <mcanal@igalia.com>
> > >>>> ---
> > >>>>    mm/huge_memory.c | 4 ++--
> > >>>>    1 file changed, 2 insertions(+), 2 deletions(-)
> > >>>>
> > >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > >>>> index f92068864469..8f41a694433c 100644
> > >>>> --- a/mm/huge_memory.c
> > >>>> +++ b/mm/huge_memory.c
> > >>>> @@ -989,7 +989,7 @@ static int __init setup_thp_anon(char *str)
> > >>>>
> > >>>>           if (!str || strlen(str) + 1 > PAGE_SIZE)
> > >>>>                   goto err;
> > >>>> -       strcpy(str_dup, str);
> > >>>> +       strscpy(str_dup, str);
> > >>>
> > >>> What is the difference between strcpy and strscpy without a size parameter?
> > >>>
> > >>> we have already a check and goto err. strcpy() is entirely safe.
> > >>>            if (!str || strlen(str) + 1 > PAGE_SIZE)
> > >>>                    goto err;
> > >>>
> > >>> My understanding is that we don't need this patch.
> > >>
> > >> strcpy() is a deprecated interface [1]. From the GitHub issue I linked
> > >> in the commit description, Kees states: "A lot of kernel code is still
> > >> using strcpy(). While the CONFIG_FORTIFY_SOURCE wrapper macros tend to
> > >> make its use mostly safe, it would be nice to eliminate the function
> > >> from the kernel entirely."
> > >
> > > I don't see any value added here since strscpy() has no size parameter and
> >
> > As `str_dup` is a sized buffer, we don't need to specify the size
> > parameter.
> >
> > > we have checked strlen(str) + 1 > PAGE_SIZE to avoid parsing any pointless
> > > bootcmd longer than PAGE_SIZE.
> >
> >  From my point of view, this is an extra layer of safety. A developer
> > could change `str_dup` to SZ_4K and leave PAGE_SIZE in the check. This
> > could pass by in a review and we would have a check that allows strings
> > bigger than the destination buffer in systems using 16KB pages.
> >
> > But see that `split_huge_pages_write` uses `strcpy` without any checks.
> >
> > I can remove the internal check from `setup_thp_anon` if you feel it
> > would be more suitable.
>
> My point is that we shouldn't remove the strlen(str) + 1 > PAGE_SIZE
> check, as it could lead to situations like this:
>
> For thp_anon=AB, where len(A) = PAGE_SIZE and B is illegal, we would
> mistakenly interpret the string as valid by ignoring B after trimming it.
>
> Therefore, if you remove the check, you could instead do:
>
> len = strscpy(str_dup, src);
> if (len >= sizeof(str_dup))
>       err;

Perhaps -E2BIG ( or < 0) i'm not quite sure :-)

>
> I have no objection to replacing strcpy with strscpy, so feel free to go
> ahead :-)
>
> >
> > Best Regards,
> > - Maíra
> >
> >  > > But I have no objection to this patch if other people like it.
> > >
> > >>
> > >> [1] https://www.kernel.org/doc/html/latest/process/deprecated.html#strcpy
> > >>
> > >> Best Regards,
> > >> - Maíra
> > >>
> > >>>
> > >>>>
> > >>>>           always = huge_anon_orders_always;
> > >>>>           madvise = huge_anon_orders_madvise;
> > >>>> @@ -4175,7 +4175,7 @@ static ssize_t split_huge_pages_write(struct file *file, const char __user *buf,
> > >>>>
> > >>>>                   tok = strsep(&buf, ",");
> > >>>>                   if (tok) {
> > >>>> -                       strcpy(file_path, tok);
> > >>>> +                       strscpy(file_path, tok);
> > >>>>                   } else {
> > >>>>                           ret = -EINVAL;
> > >>>>                           goto out;
> > >>>> --
> > >>>> 2.46.2
> > >>>>
> > >>>
> Thanks
> barry