On Mon, Jan 18, 2021 at 10:22:56PM +0100, Bodo Stroesser wrote: > 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++; It is necessary to properly check for overflow, because the easy to understand check doesn't prove that round_up will work, only that >> results in something that fits in an int and that +1 won't overflow the int. > Wouldn't it be better to keep the original line instead: > > nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order); This can overflow inside the round_up Jason
