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SubjectRe: [PATCH] mm/usercopy: Use memory range to be accessed for wraparound check
On 2018-11-14 03:46, William Kucharski wrote:
>> On Nov 14, 2018, at 4:09 AM, David Laight <David.Laight@ACULAB.COM>
>> wrote:
>>
>> From: William Kucharski
>>> Sent: 14 November 2018 10:35
>>>
>>>> On Nov 13, 2018, at 5:51 PM, Isaac J. Manjarres
>>>> <isaacm@codeaurora.org> wrote:
>>>>
>>>> diff --git a/mm/usercopy.c b/mm/usercopy.c
>>>> index 852eb4e..0293645 100644
>>>> --- a/mm/usercopy.c
>>>> +++ b/mm/usercopy.c
>>>> @@ -151,7 +151,7 @@ static inline void check_bogus_address(const
>>>> unsigned long ptr, unsigned long n,
>>>> bool to_user)
>>>> {
>>>> /* Reject if object wraps past end of memory. */
>>>> - if (ptr + n < ptr)
>>>> + if (ptr + (n - 1) < ptr)
>>>> usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
>>>
>>> I'm being paranoid, but is it possible this routine could ever be
>>> passed "n" set to zero?
>>>
>>> If so, it will erroneously abort indicating a wrapped address as (n -
>>> 1) wraps to ULONG_MAX.
>>>
>>> Easily fixed via:
>>>
>>> if ((n != 0) && (ptr + (n - 1) < ptr))
>>
>> Ugg... you don't want a double test.
>>
>> I'd guess that a length of zero is likely, but a usercopy that
>> includes
>> the highest address is going to be invalid because it is a kernel
>> address
>> (on most archs, and probably illegal on others).
>> What you really want to do is add 'ptr + len' and check the carry
>> flag.
>
> The extra test is only a few extra instructions, but I understand the
> concern. (Though I don't
> know how you'd access the carry flag from C in a machine-independent
> way. Also, for the
> calculation to be correct you still need to check 'ptr + (len - 1)'
> for the wrap.)
>
> You could also theoretically call gcc's __builtin_uadd_overflow() if
> you want to get carried away.
>
> As I mentioned, I was just being paranoid, but the passed zero length
> issue stood out to me.
>
> William Kucharski

Hi William,

Thank you and David for your feedback. The check_bogus_address() routine
is only invoked from one place in the kernel, which is
__check_object_size(). Before invoking check_bogus_address,
__check_object_size ensures that n is non-zero, so it is not possible to
call this routine with n being 0. Therefore, we shouldn't run into the
scenario you described. Also, in the case where we are copying a page's
contents into a kernel space buffer and will not have that buffer
interacting with userspace at all, this change to that check should
still be valid, correct?

Thanks,
Isaac Manjarres

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