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 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);

Needs to be checked, add a precondition to order does not help. I
already proposed a straightforward algorithm you can use.

Jason