Re: [RFC PATCH 2/7] x86/sci: add core implementation for system call isolation

[Ingo Molnar <mingo@kernel.org>]

* Andy Lutomirski <luto@kernel.org> wrote:

> On Sat, Apr 27, 2019 at 3:46 AM Ingo Molnar <mingo@kernel.org> wrote:

> > So I'm wondering whether there's a 4th choice as well, which avoids
> > control flow corruption *before* it happens:
> >
> >  - A C language runtime that is a subset of current C syntax and
> >    semantics used in the kernel, and which doesn't allow access outside
> >    of existing objects and thus creates a strictly enforced separation
> >    between memory used for data, and memory used for code and control
> >    flow.
> >
> >  - This would involve, at minimum:
> >
> >     - tracking every type and object and its inherent length and valid
> >       access patterns, and never losing track of its type.
> >
> >     - being a lot more organized about initialization, i.e. no
> >       uninitialized variables/fields.
> >
> >     - being a lot more strict about type conversions and pointers in
> >       general.
> 
> You're not the only one to suggest this.  There are at least a few
> things that make this extremely difficult if not impossible.  For
> example, consider this code:
> 
> void maybe_buggy(void)
> {
>   int a, b;
>   int *p = &a;
>   int *q = (int *)some_function((unsigned long)p);
>   *q = 1;
> }
> 
> If some_function(&a) returns &a, then all is well.  But if
> some_function(&a) returns &b or even a valid address of some unrelated
> kernel object, then the code might be entirely valid and correct C,
> but I don't see how the runtime checks are supposed to tell whether
> the resulting address is valid or is a bug.  This type of code is, I
> think, quite common in the kernel -- it happens in every data
> structure where we have unions of pointers and integers or where we
> steal some known-zero bits of a pointer to store something else.

So the thing is, for the infinitely large state space of "valid C code" 
we already disallow an infinitely many versions in the Linux kernel.

We have complicated rules that disallow certain C syntactical and 
semantical constructs, both on the tooling (build failure/warning) and on 
the review (style/taste) level.

So the question IMHO isn't whether it's "valid C", because we already 
have the Linux kernel's own C syntax variant and are enforcing it with 
varying degrees of success.

The question is whether the example you gave can be written in a strongly 
typed fashion, whether it makes sense to do so, and what the costs are.

I think it's evident that it can be written with strongly typed 
constructs, by separating pointers from embedded error codes - with 
negative side effects to code generation: for example it increases 
structure sizes and error return paths.

I think there's four main costs of converting such a pattern to strongly 
typed constructs:

 - memory/cache footprint:  there's a nonzero cost there.
 - performance:             this will hurt too.
 - code readability:        this will probably improve.
 - code robustness:         this will improve too.

So I think the proper question to ask is not whether there's common C 
syntax within the kernel that would have to be rewritten, but whether the 
total sum of memory and runtime overhead of strongly typed C programming 
(if it's possible/desirable) is larger than the total sum of a typical 
Linux distro enabling the various current and proposed kernel hardening 
features that have a runtime overhead:

 - the SMAP/SMEP overhead of STAC/CLAC for every single user copy

 - other usercopy hardening features

 - stackprotector

 - KASLR

 - compiler plugins against information leaks

 - proposed KASLR extension to implement module randomization and -PIE overhead

 - proposed function call integrity checks

 - proposed per system call kernel stack offset randomization

 - ( and I'm sure I forgot about a few more, and it's all still only 
     reactive security, not proactive security. )

That's death by a thousand cuts and CR3 switching during system calls is 
also throwing a hand grenade into the fight ;-)

So if people are also proposing to do CR3 switches in every system call, 
I'm pretty sure the answer is "yes, even a managed C runtime is probably 
faster than *THAT* sum of a performanc mess" - at least with the current 
CR3 switching x86-uarch cost structure...

Thanks,

	Ingo