On Thu, Apr 11, 2019 at 1:39 AM Alexander Potapenko <glider@xxxxxxxxxx> wrote: > > On Wed, Apr 10, 2019 at 6:09 PM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > On Wed, Apr 10, 2019 at 6:18 AM Alexander Potapenko <glider@xxxxxxxxxx> wrote: > > > > > > This config option adds the possibility to initialize newly allocated > > > pages and heap objects with a 0xAA pattern. > > > There's already a number of places where allocations are initialized > > > based on the presence of __GFP_ZERO flag. We just change this code so > > > that under CONFIG_INIT_ALL_HEAP these allocations are always initialized > > > with either 0x00 or 0xAA depending on the __GFP_ZERO. > > > > Why not just make __GFP_ZERO unconditional instead? This looks like > > it'd be simpler and not need arch-specific implementation? > > Right, but it would mean we can only initialize with 0x00 pattern. > I believe that for testing purposes a nonzero pattern is better, Can it be implemented in a way that isn't arch-specific? I'd really like to have a general solution that works immediately for all architectures. (Can't everything just use a memset?) > because it'll not only assure the execution is deterministic, but will > also uncover logic bugs earlier (see the discussion at > https://reviews.llvm.org/D54604?id=174471) > For hardening purposes the pattern shouldn't matter much. So, for hardening, it actually does matter but only in certain cases. On 64-bit, a 0xAA... pointer will have the high bit set, so it'll land in the non-canonical memory range, which is good. For 32-bit, 0xAA... will be in userspace (TASK_SIZE is 0xC0000000). In the above URL I see now that 32-bit pointer init gets 0x000000AA, which is good, but for heap init, types aren't known. So perhaps use 0x000000AA for 32-bit and 0xAA... for 64-bit heap init? (0xAA... has stronger properties since there have been NULL page mapping bypass flaws in the (recent!) past, so I could see keeping that for 64-bit instead of just using 0-init everywhere.) > If you think arch-specific code is too much of a trouble, we could > implement clear_page_pattern() using memset() on every architecture, > but allow the user to choose between slow (0xAA) and production (0x00) > modes. How about 32-bit use 0x00, 64-bit use 0xAA (and provide per-arch speed-ups with a generic "slow" version for all the other architectures?) -Kees -- Kees Cook
