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

[Bodo Stroesser]

On 19.01.21 00:48, Jason Gunthorpe wrote:
> 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

I had a second look into math.h, but I don't find any reason why 
round_up could overflow. Can you give a hint please?

Regarding the overflow checks: would it be a good idea to not check
length >> (PAGE_SHIFT + order) in the beginning, but check nalloc
immediately before the kmalloc_array() as the only overrun check:

	if ((unsigned long long)nalloc << (PAGE_SHIFT + order) < length)
		return NULL;

-Bodo