Re: How does swsusp work with randomization features? (was: mm/slab: Initialise random_kmalloc_seed after initcalls)

["Rafael J. Wysocki" <rafael@kernel.org>]

On Wed, Feb 19, 2025 at 6:25 PM Kees Cook <kees@kernel.org> wrote:
>
> On Fri, Feb 14, 2025 at 09:44:59PM +0900, Harry (Hyeonggon) Yoo wrote:
> > On Fri, Feb 14, 2025 at 06:02:52PM +0800, Huacai Chen wrote:
> > > On Fri, Feb 14, 2025 at 5:33 PM Harry (Hyeonggon) Yoo
> > > <42.hyeyoo@gmail.com> wrote:
> > > >
> > > > On Thu, Feb 13, 2025 at 11:20:22AM +0800, Huacai Chen wrote:
> > > > > Hi, Harry,
> > > > >
> > > > > On Wed, Feb 12, 2025 at 11:39 PM Harry (Hyeonggon) Yoo
> > > > > <42.hyeyoo@gmail.com> wrote:
> > > > > > On Wed, Feb 12, 2025 at 11:17 PM Huacai Chen <chenhuacai@loongson.cn> wrote:
> > > > > > >
> > > > > > > Hibernation assumes the memory layout after resume be the same as that
> > > > > > > before sleep, but CONFIG_RANDOM_KMALLOC_CACHES breaks this assumption.
> > > > > >
> > > > > > Could you please elaborate what do you mean by
> > > > > > hibernation assumes 'the memory layout' after resume be the same as that
> > > > > > before sleep?
> > > > > >
> > > > > > I don't understand how updating random_kmalloc_seed breaks resuming from
> > > > > > hibernation. Changing random_kmalloc_seed affects which kmalloc caches
> > > > > > newly allocated objects are from, but it should not affect the objects that are
> > > > > > already allocated (before hibernation).
> > > > >
> > > > > When resuming, the booting kernel should switch to the target kernel,
> > > > > if the address of switch code (from the booting kernel) is the
> > > > > effective data of the target kernel, then the switch code may be
> > > > > overwritten.
> > > >
> > > > Hmm... I'm still missing some pieces.
> > > > How is the kernel binary overwritten when slab allocations are randomized?
> > > >
> > > > Also, I'm not sure if it's even safe to assume that the memory layout is the
> > > > same across boots. But I'm not an expert on swsusp anyway...
> > > >
> > > > It'd be really helpful for linux-pm folks to clarify 1) what are the
> > > > (architecture-independent) assumptions are for swsusp to work, and
> > > > 2) how architectures dealt with other randomization features like kASLR...
> > >
> >
> > [+Cc few more people that worked on slab hardening]
> >
> > > I'm sorry to confuse you. Binary overwriting is indeed caused by
> > > kASLR, so at least on LoongArch we should disable kASLR for
> > > hibernation.
> >
> > Understood.
> >
> > > Random kmalloc is another story, on LoongArch it breaks smpboot when
> > > resuming, the details are:
> > > 1, LoongArch uses kmalloc() family to allocate idle_task's
> > > stack/thread_info and other data structures.
> > > 2, If random kmalloc is enabled, idle_task's stack in the booting
> > > kernel may be other things in the target kernel.
> >
> > Slab hardening features try so hard to prevent such predictability.
> > For example, SLAB_FREELIST_RANDOM could also randomize the address
> > kmalloc objects are allocated at.
> >
> > Rather than hacking CONFIG_RANDOM_KMALLOC_CACHES like this, we could
> > have a single option to disable slab hardening features that makes
> > the address unpredictable.
> >
> > It'd be nice to have something like ARCH_SUPPORTS_SLAB_RANDOM which
> > some hardening features depend on. And then let some arches conditionally
> > not select ARCH_SUPPORTS_SLAB_RANDOM if hibernation's enabled
> > (at cost of less hardening)?
>
> I find this whole thread confusing. :) Hibernation should already do
> whatever it need to to get out of the way of the kernel it is restoring
> to memory. The random locations shouldn't matter at all: they're all
> stored in the image. I am not a hibernation expert, but my understanding
> is that the "resume" kernel moves itself out of the way to restore the
> KASLR-ed hibernation image and puts everything back exactly as it was.
> Randomization should not matter at all: it's just simply "put everything
> back where it was".

Exactly.

> Yes, the tricky part is the "move itself out of the way", but that's
> required for any kernel that support being relocatable (a prerequisite
> for KASLR), and KASLR is just an aggressive form of "the relocatable
> kernel might be anywhere" beyond just different boot loaders putting it
> in a handful of different potential offsets.

Right.

Thanks!