On Mon, Sep 26, 2022 at 09:11:24PM +0200, Andrey Konovalov wrote: > On Tue, Sep 13, 2022 at 8:54 AM Feng Tang <feng.tang@xxxxxxxxx> wrote: > > > > Hi Feng, > > > kzalloc/kmalloc will round up the request size to a fixed size > > (mostly power of 2), so the allocated memory could be more than > > requested. Currently kzalloc family APIs will zero all the > > allocated memory. > > > > To detect out-of-bound usage of the extra allocated memory, only > > zero the requested part, so that sanity check could be added to > > the extra space later. > > I still don't like the idea of only zeroing the requested memory and > not the whole object. Considering potential info-leak vulnerabilities. I really really do not like reducing the zeroing size. We're trying to be proactive against _flaws_, which means that when there's a memory over-read (or uninitialized use), suddenly the scope of the exposure (or control) is wider/looser. Imagine the (unfortunately very common) case of use-after-free attacks, which leverage type confusion: some object is located in kmalloc-128 because it's 126 bytes. That slot gets freed and reallocated to, say, a 97 byte object going through kzalloc() or zero-on-init. With this patch the bytes above the 97 don't get zeroed, and the stale data from the prior 126 byte object say there happily to be used again later through a dangling pointer, or whatever. Without the proposed patch, the entire 128 bytes is wiped, which makes stale data re-use more difficult. > > Performance wise, smaller zeroing length also brings shorter > > execution time, as shown from test data on various server/desktop > > platforms. For these cases, I think a much better solution is to provide those sensitive allocations their own dedicated kmem_cache. > > > > For kzalloc users who will call ksize() later and utilize this > > extra space, please be aware that the space is not zeroed any > > more. > > CC Kees Thanks! Well, the good news is that ksize() side-effects is hopefully going to vanish soon, but my objections about stale memory remain. -Kees -- Kees Cook
