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

[Jason Gunthorpe <jgg@ziepe.ca>]

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