On Thu, Jun 04, 2020 at 06:23:38PM +0300, Alexander Popov wrote: > On 04.06.2020 17:01, Jann Horn wrote: > > On Thu, Jun 4, 2020 at 3:51 PM Alexander Popov <alex.po...@linux.com> wrote: > >> Some time ago Variable Length Arrays (VLA) were removed from the kernel. > >> The kernel is built with '-Wvla'. Let's exclude alloca() from the > >> instrumentation logic and make it simpler. The build-time assertion > >> against alloca() is added instead. > > [...] > >> + /* Variable Length Arrays are forbidden in the > >> kernel */ > >> + gcc_assert(!is_alloca(stmt)); > > > > There is a patch series from Elena and Kees on the kernel-hardening > > list that deliberately uses __builtin_alloca() in the syscall entry > > path to randomize the stack pointer per-syscall - see > > <https://lore.kernel.org/kernel-hardening/20200406231606.37619-4-keesc...@chromium.org/>. > > Thanks, Jann. > > At first glance, leaving alloca() handling in stackleak instrumentation logic > would allow to integrate stackleak and this version of random_kstack_offset.
Right, it seems there would be a need for this coverage to remain, otherwise the depth of stack erasure might be incorrect. It doesn't seem like the other patches strictly depend on alloca() support being removed, though? > Kees, Elena, did you try random_kstack_offset with upstream stackleak? I didn't try that combination yet, no. It seemed there would likely still be further discussion about the offset series first (though the thread has been silent -- I'll rebase and resend it after rc2). > It looks to me that without stackleak erasing random_kstack_offset can be > weaker. I mean, if next syscall has a bigger stack randomization gap, the data > on thread stack from the previous syscall is not overwritten and can be used. > Am > I right? That's correct. I think the combination is needed, but I don't think they need to be strictly tied together. > Another aspect: CONFIG_STACKLEAK_METRICS can be used for guessing kernel stack > offset, which is bad. It should be disabled if random_kstack_offset is on. Agreed. -- Kees Cook
