From: Ingo Molnar <mingo@kernel.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>,
LKML <linux-kernel@vger.kernel.org>,
Alexandre Chartre <alexandre.chartre@oracle.com>,
Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
James Bottomley <James.Bottomley@hansenpartnership.com>,
Jonathan Adams <jwadams@google.com>,
Kees Cook <keescook@chromium.org>, Paul Turner <pjt@google.com>,
Peter Zijlstra <peterz@infradead.org>,
Thomas Gleixner <tglx@linutronix.de>,
Linux-MM <linux-mm@kvack.org>,
LSM List <linux-security-module@vger.kernel.org>,
X86 ML <x86@kernel.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC PATCH 2/7] x86/sci: add core implementation for system call isolation
Date: Tue, 30 Apr 2019 07:03:37 +0200 [thread overview]
Message-ID: <20190430050336.GA92357@gmail.com> (raw)
In-Reply-To: <CALCETrUn_86VAd8FGacJ169xcWE6XQngAMMhvgd1Aa6ZxhGhtA@mail.gmail.com>
* 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
next prev parent reply other threads:[~2019-04-30 5:03 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-25 21:45 [RFC PATCH 0/7] x86: introduce system calls addess space isolation Mike Rapoport
2019-04-25 21:45 ` [RFC PATCH 1/7] x86/cpufeatures: add X86_FEATURE_SCI Mike Rapoport
2019-04-25 21:45 ` [RFC PATCH 2/7] x86/sci: add core implementation for system call isolation Mike Rapoport
2019-04-26 7:49 ` Peter Zijlstra
2019-04-28 5:45 ` Mike Rapoport
2019-04-26 8:31 ` Ingo Molnar
2019-04-26 9:58 ` Ingo Molnar
2019-04-26 21:26 ` Andy Lutomirski
2019-04-27 8:47 ` Ingo Molnar
2019-04-27 10:46 ` Ingo Molnar
2019-04-29 18:26 ` James Morris
2019-04-29 18:43 ` Andy Lutomirski
2019-04-29 18:46 ` Andy Lutomirski
2019-04-30 5:03 ` Ingo Molnar [this message]
2019-04-30 9:38 ` Peter Zijlstra
2019-04-30 11:05 ` Ingo Molnar
2019-05-02 11:35 ` Robert O'Callahan
2019-05-02 15:20 ` Ingo Molnar
2019-05-02 21:07 ` Robert O'Callahan
2019-04-26 14:44 ` James Bottomley
2019-04-26 14:46 ` Dave Hansen
2019-04-26 14:57 ` James Bottomley
2019-04-26 15:07 ` Andy Lutomirski
2019-04-26 15:19 ` James Bottomley
2019-04-26 17:40 ` Andy Lutomirski
2019-04-26 18:49 ` James Bottomley
2019-04-26 19:22 ` Andy Lutomirski
2019-04-25 21:45 ` [RFC PATCH 3/7] x86/entry/64: add infrastructure for switching to isolated syscall context Mike Rapoport
2019-04-25 21:45 ` [RFC PATCH 4/7] x86/sci: hook up isolated system call entry and exit Mike Rapoport
2019-04-25 21:45 ` [RFC PATCH 5/7] x86/mm/fault: hook up SCI verification Mike Rapoport
2019-04-26 7:42 ` Peter Zijlstra
2019-04-28 5:47 ` Mike Rapoport
2019-04-30 16:44 ` Andy Lutomirski
2019-05-01 5:39 ` Mike Rapoport
2019-04-25 21:45 ` [RFC PATCH 6/7] security: enable system call isolation in kernel config Mike Rapoport
2019-04-25 21:45 ` [RFC PATCH 7/7] sci: add example system calls to exercse SCI Mike Rapoport
2019-04-26 0:30 ` [RFC PATCH 0/7] x86: introduce system calls addess space isolation Andy Lutomirski
2019-04-26 8:07 ` Jiri Kosina
2019-04-28 6:01 ` Mike Rapoport
2019-04-26 14:41 ` Dave Hansen
2019-04-28 6:08 ` Mike Rapoport
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