REDHAT-BUG-2507213: Medium severity Linux Linux kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved:

mm/slab: do not limit zeroing to origsize when only red zoning is enabled

When init (zeroing) on allocation is requested, for kmalloc() we generally have to zero the full object size even if a smaller size is requested, in order to provide krealloc()'s GFPZERO guarantees.

But if we track the requested size, krealloc() uses that information to do the right thing, so we can zero only the requested size. With red zoning also enabled, any extra size became part of the red zone, so it must not be zeroed and thus we must zero only the requested size.

However the current check is imprecise, and will trigger also when only SLABREDZONE is enabled without SLABSTOREUSER (which enables tracking the requested size). This means enabling red zoning alone can compromise krealloc()'s GFPZERO contract.

Fix this by using slubdebugorigsize() instead, which is the exact check for whether the requested size is tracked. We don't need to care if red zoning is also enabled or not. Also update and expand the comment accordingly.