Re: [PATCH 10/11] scs: generic scs code updated to leverage hw assisted shadow stack

[Deepak Gupta]

On Fri, Jul 25, 2025 at 06:05:22PM +0000, Edgecombe, Rick P wrote:
> On Fri, 2025-07-25 at 10:19 -0700, Deepak Gupta wrote:
> > > This doesn't update the direct map alias I think. Do you want to protect it?
> >
> > Yes any alternate address mapping which is writeable is a problem and dilutes
> > the mechanism. How do I go about updating direct map ? (I pretty new to linux
> > kernel and have limited understanding on which kernel api's to use here to
> > unmap
> > direct map)
>
> Here is some info on how it works:
>
> set_memory_foo() variants should (I didn't check riscv implementation, but on
> x86) update the target addresses passed in *and* the direct map alias. And flush
> the TLB.
>
> vmalloc_node_range() will just set the permission on the vmalloc alias and not
> touch the direct map alias.
>
> vfree() works by trying to batch the flushing for unmap operations to avoid
> flushing the TLB too much. When memory is unmapped in userspace, it will only
> flush on the CPU's with that MM (process address space). But for kernel memory
> the mappings are shared between all CPUs. So, like on a big server or something,
> it requires way more work and distance IPIs, etc. So vmalloc will try to be
> efficient and keep zapped mappings unflushed until it has enough to clean them
> up in bulk. In the meantime it won't reuse that vmalloc address space.
>
> But this means there can also be other vmalloc aliases still in the TLB for any
> page that gets allocated from the page allocator. If you want to be fully sure
> there are no writable aliases, you need to call vm_unmap_aliases() each time you
> change kernel permissions, which will do the vmalloc TLB flush immediately. Many
> set_memory() implementations call this automatically, but it looks like not
> riscv.
>
>
> So doing something like vmalloc(), set_memory_shadow_stack() on alloc and
> set_memory_rw(), vfree() on free is doing the expensive flush (depends on the
> device how expensive) in a previously fast path. Ignoring the direct map alias
> is faster. A middle ground would be to do the allocation/conversion and freeing
> of a bunch of stacks at once, and recycle them.
>
>
> You could make it tidy first and then optimize it later, or make it faster first
> and maximally secure later. Or try to do it all at once. But there have long
> been discussions on batching type kernel memory permission solutions. So it
> would could be a whole project itself.

Thanks Rick. Another approach I am thinking could be making vmalloc
intrinsically aware of certain range to be security sensitive. Meaning during
vmalloc initialization itself, it could reserve a range which is ensured to be
not direct mapped. Whenever `PAGE_SHADOWSTACK` is requested, it always comes
from this range (which is guaranteed to be never direct mapped).

I do not expect hardware assisted shadow stack to be more than 4K in size
(should support should 512 call-depth). A system with 30,000 active threads
(taking a swag number here), will need 30,000 * 2 (one for guard) = 60000 pages.
That's like ~245 MB address range. We can be conservative and have 1GB range in
vmalloc larger range reserved for shadow stack. vmalloc ensures that this
range's direct mappping always have read-only encoding in ptes. Sure this number
(shadow stack range in larget vmalloc range) could be configured so that user
can do their own trade off.

Does this approach look okay?

> > >
> > > >
> > > >   out:
> > > > @@ -59,7 +72,7 @@ void *scs_alloc(int node)
> > > >   	if (!s)
> > > >   		return NULL;
> > > >
> > > > -	*__scs_magic(s) = SCS_END_MAGIC;
> > > > +	__scs_store_magic(__scs_magic(s), SCS_END_MAGIC);
> > > >
> > > >   	/*
> > > >   	 * Poison the allocation to catch unintentional accesses to
> > > > @@ -87,6 +100,16 @@ void scs_free(void *s)
> > > >   			return;
> > > >
> > > >   	kasan_unpoison_vmalloc(s, SCS_SIZE, KASAN_VMALLOC_PROT_NORMAL);
> > > > +	/*
> > > > +	 * Hardware protected shadow stack is not writeable by regular
> > > > stores
> > > > +	 * Thus adding this back to free list will raise faults by
> > > > vmalloc
> > > > +	 * It needs to be writeable again. It's good sanity as well
> > > > because
> > > > +	 * then it can't be inadvertently accesses and if done, it will
> > > > fault.
> > > > +	 */
> > > > +#ifdef CONFIG_ARCH_HAS_KERNEL_SHADOW_STACK
> > > > +	set_memory_rw((unsigned long)s, (SCS_SIZE/PAGE_SIZE));
> > >
> > > Above you don't update the direct map permissions. So I don't think you need
> > > this. vmalloc should flush the permissioned mapping before re-using it with
> > > the
> > > lazy cleanup scheme.
> >
> > If I didn't do this, I was getting a page fault on this vmalloc address. It
> > directly
> > uses first 8 bytes to add it into some list and that was the location of
> > fault.
>
> Ah right! Because it is using the vfree atomic variant.
>
> You could create your own WQ in SCS and call vfree() in non-atomic context. If
> you want to avoid thr set_memory_rw() on free, in the ignoring the direct map
> case.