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

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

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;

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