On Tue, Sep 27, 2022 at 04:15:02AM +0800, Kees Cook wrote: > 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. Thanks for the details explaination, which is a valid concern. And Andrey's suggestion is a good solution: only reduce the zeroing size for kmalloc-redzone enabled objects, as the extra space will be redzoned, and no info will be leaked. Thanks, Feng
