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

["Maíra Canal" <mcanal@igalia.com>]

+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.

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