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 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