Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free() warning

Bodo Stroesser
2021-01-18T21:26:23Z

On 18.01.21 21:24, Jason Gunthorpe wrote:
> On Mon, Jan 18, 2021 at 03:08:51PM -0500, Douglas Gilbert wrote:
>> On 2021-01-18 1:28 p.m., Jason Gunthorpe wrote:
>>> On Mon, Jan 18, 2021 at 11:30:03AM -0500, Douglas Gilbert wrote:
>>>
>>>> After several flawed attempts to detect overflow, take the fastest
>>>> route by stating as a pre-condition that the 'order' function argument
>>>> cannot exceed 16 (2^16 * 4k = 256 MiB).
>>>
>>> That doesn't help, the point of the overflow check is similar to
>>> overflow checks in kcalloc: to prevent the routine from allocating
>>> less memory than the caller might assume.
>>>
>>> For instance ipr_store_update_fw() uses request_firmware() (which is
>>> controlled by userspace) to drive the length argument to
>>> sgl_alloc_order(). If userpace gives too large a value this will
>>> corrupt kernel memory.
>>>
>>> So this math:
>>>
>>> nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order);
>>
>> But that check itself overflows if order is too large (e.g. 65).
>
> I don't reall care about order. It is always controlled by the kernel
> and it is fine to just require it be low enough to not
> overflow. length is the data under userspace control so math on it
> must be checked for overflow.
>
>> Also note there is another pre-condition statement in that function's
>> definition, namely that length cannot be 0.
>
> I don't see callers checking for that either, if it is true length 0
> can't be allowed it should be blocked in the function
>
> Jason
>

A already said, I also think there should be a check for length or
rather nent overflow.

I like the easy to understand check in your proposed code:

if (length >> (PAGE_SHIFT + order) >= UINT_MAX)
return NULL;

But I don't understand, why you open-coded the nent calculation:

nent = length >> (PAGE_SHIFT + order);
if (length & ((1ULL << (PAGE_SHIFT + order)) - 1))
nent++;

Wouldn't it be better to keep the original line instead:

nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order);

Or maybe even better:

nent = DIV_ROUND_UP(length, PAGE_SIZE << order);

I think, combining the above lines results in short and easily readable code:

u32 elem_len;

if (length >> (PAGE_SHIFT + order) >= UINT_MAX)
return NULL;
nent = DIV_ROUND_UP(length, PAGE_SIZE << order);

if (chainable) {
if (check_add_overflow(nent, 1, &nalloc))
return NULL;
}
else
nalloc = nent;

Thank you,
Bodo