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Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

From Kees Cook <keescook@chromium.org>
Newsgroups linux.kernel
Subject Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode
Date 2017-05-09 18:40 +0200
Message-ID <tFgl3-4fY-11@gated-at.bofh.it> (permalink)
References (2 earlier) <tELqV-VP-1@gated-at.bofh.it> <tELKi-14K-19@gated-at.bofh.it> <tESLN-5F6-35@gated-at.bofh.it> <tESLN-5F6-43@gated-at.bofh.it> <tF7hM-6w8-15@gated-at.bofh.it>
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On Mon, May 8, 2017 at 11:56 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Kees Cook <keescook@chromium.org> wrote:
>
>> > There's the option of using GCC plugins now that the infrastructure was
>> > upstreamed from grsecurity. It can be used as part of the regular build
>> > process and as long as the analysis is pretty simple it shouldn't hurt compile
>> > time much.
>>
>> Well, and that the situation may arise due to memory corruption, not from
>> poorly-matched set_fs() calls, which static analysis won't help solve. We need
>> to catch this bad kernel state because it is a very bad state to run in.

[attempting some thread-merging]

> Ok, so that's CVE-2010-4258, where an oops with KERNEL_DS set was used to escalate
> privileges, due to the kernel's oops handler not cleaning up the KERNEL_DS. The
> exploit used another bug, a crash in a network protocol handler, to execute the
> oops handler with KERNEL_DS set.

Right, I didn't mean to suggest that vulnerability would be fixed by
this solution. I was trying to show how there can be some pretty
complex interaction with exceptions/interrupts/etc that would make
pure static analysis still miss things.

> If memory corruption corrupted the task state into having addr_limit set to
> KERNEL_DS then there's already a fair chance that it's game over: it could also
> have set *uid to 0, or changed a sensitive PF_ flag, or a number of other
> things...
>
> Furthermore, think about it: there's literally an infinite amount of corrupted
> task states that could be a security problem and that could be checked after every
> system call. Do we want to check every one of them?

Right, but this "slippery slope" argument isn't the best way to reject
security changes. Let me take a step back and describe the threat, and
where we should likely spend time:

The primary threat with addr_limit getting changed is that a
narrowly-scoped attack (traditionally stack exhaustion or
adjacent-stack large-index writes) could be leveraged into opening the
entire kernel to writes (by allowing all syscalls with a
copy_to_user() call to suddenly be able to write to kernel memory).
So, really, the flaw is having addr_limit at all. Removing set_fs()
should, I think, allow this to become a const (or at least should get
us a lot closer).

The main path to corrupting addr_limit has been via stack corruption.
On architectures with CONFIG_THREAD_INFO_IN_TASK, this risk is greatly
reduced already, but it's not universally available yet. (And as long
as we're talking about stack attacks, CONFIG_VMAP_STACK makes
cross-stack overflows go away, and cross-stack indexing harder, but
that's not really about addr_limit since currently nothing with
VMAP_STACK doesn't already have THREAD_INFO_IN_TASK.)

So, left with a still exploitable target in memory that allows such an
expansion of attack method, I still think it's worth keeping this
patch series, but if we can drop set_fs() I could probably be
convinced the benefit of the series doesn't exceed the cost on
THREAD_INFO_IN_TASK-architectures (x86, arm64, s390). But that means
at least currently keeping it on arm, for example. If we can make
addr_limit const, well, we don't need the series at all.

-Kees

-- 
Kees Cook
Pixel Security

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[PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode Thomas Garnier <thgarnie@google.com> - 2017-04-28 17:40 +0200
  Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode Thomas Garnier <thgarnie@google.com> - 2017-05-06 00:20 +0200
    Re: [PATCH v9 1/4] syscalls: Verify address limit before returning  to user-mode Ingo Molnar <mingo@kernel.org> - 2017-05-08 09:40 +0200
      Re: [PATCH v9 1/4] syscalls: Verify address limit before returning  to user-mode Ingo Molnar <mingo@kernel.org> - 2017-05-08 10:00 +0200
        Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Daniel Micay <danielmicay@gmail.com> - 2017-05-08 17:30 +0200
          Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Kees Cook <keescook@chromium.org> - 2017-05-08 17:30 +0200
            Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Thomas Garnier <thgarnie@google.com> - 2017-05-08 22:00 +0200
            Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Ingo Molnar <mingo@kernel.org> - 2017-05-09 09:00 +0200
              Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Greg KH <greg@kroah.com> - 2017-05-09 13:20 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Thomas Garnier <thgarnie@google.com> - 2017-05-09 16:40 +0200
              Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Kees Cook <keescook@chromium.org> - 2017-05-09 18:40 +0200
      Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Greg KH <greg@kroah.com> - 2017-05-08 14:50 +0200
        Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Ingo Molnar <mingo@kernel.org> - 2017-05-09 08:50 +0200
          Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-09 11:00 +0200
            Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Andy Lutomirski <luto@kernel.org> - 2017-05-09 15:10 +0200
              Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-09 15:10 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-09 18:10 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Kees Cook <keescook@chromium.org> - 2017-05-09 19:00 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Andy Lutomirski <luto@amacapital.net> - 2017-05-10 01:00 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Kees Cook <keescook@chromium.org> - 2017-05-10 01:40 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Andy Lutomirski <luto@kernel.org> - 2017-05-10 04:10 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 09:20 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Borislav Petkov <bp@alien8.de> - 2017-05-11 13:30 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 08:50 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 04:20 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 04:50 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 05:20 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 05:30 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 05:50 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 09:00 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 09:00 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 09:30 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 09:40 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 09:00 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Arnd Bergmann <arnd@arndb.de> - 2017-05-10 09:30 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 09:40 +0200
              Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Brian Gerst <brgerst@gmail.com> - 2017-05-09 18:10 +0200
              Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Arnd Bergmann <arnd@arndb.de> - 2017-05-10 09:40 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-10 10:10 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Christoph Hellwig <hch@infradead.org> - 2017-05-10 10:20 +0200
                Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address  limit before returning to user-mode Andy Lutomirski <luto@kernel.org> - 2017-05-11 02:20 +0200
      Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode Kees Cook <keescook@chromium.org> - 2017-05-08 15:20 +0200
        Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode Jann Horn <jannh@google.com> - 2017-05-08 16:10 +0200
          Re: [PATCH v9 1/4] syscalls: Verify address limit before returning  to user-mode Al Viro <viro@ZenIV.linux.org.uk> - 2017-05-08 22:50 +0200
        Re: [PATCH v9 1/4] syscalls: Verify address limit before returning  to user-mode Ingo Molnar <mingo@kernel.org> - 2017-05-08 16:10 +0200
          Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode Kees Cook <keescook@chromium.org> - 2017-05-08 17:30 +0200
            Re: [PATCH v9 1/4] syscalls: Verify address limit before returning  to user-mode Ingo Molnar <mingo@kernel.org> - 2017-05-09 08:40 +0200

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