linux-kernel - Re: [PATCH 0/7] x86: uaccess hardening, easy part

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Date:	Tue, 24 May 2016 23:55:07 -0400
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH 0/7] x86: uaccess hardening, easy part

On Tue, May 24, 2016 at 6:48 PM, Andy Lutomirski <luto@...nel.org> wrote:
> This series hardens x86's uaccess code a bit.  It adds warnings for
> some screwups, adds an OOPS for a major exploitable screwup, and it
> improves debuggability a bit by indicating non-default fs in oopses.
>
> It shouldn't cause any new OOPSes except in the particularly
> dangerous case where the kernel faults on a kernel address under
> USER_DS, which indicates that an access_ok is missing and is likely
> to be easily exploitable -- OOPSing will make it harder to exploit.
>
> I have some draft patches to force OOPSes on user address accesses
> under KERNEL_DS (which is a big no-no), but I'd rather make those
> warn instead of OOPSing, and I don't have a good implementation of
> that yet.  Those patches aren't part of this series.
>
> Andy Lutomirski (7):
>   x86/xen: Simplify set_aliased_prot
>   x86/extable: Pass error_code and an extra unsigned long to exhandlers
>   x86/uaccess: Give uaccess faults their own handler
>   x86/dumpstack: If addr_limit is non-default, display it
>   x86/uaccess: Warn on uaccess faults other than #PF
>   x86/uaccess: Don't fix up USER_DS uaccess faults to kernel addresses
>   x86/uaccess: OOPS or warn on a fault with KERNEL_DS and
>     !pagefault_disabled()
>
>  arch/x86/include/asm/uaccess.h   |  19 ++++---
>  arch/x86/kernel/cpu/mcheck/mce.c |   2 +-
>  arch/x86/kernel/dumpstack_32.c   |   4 ++
>  arch/x86/kernel/dumpstack_64.c   |   5 ++
>  arch/x86/kernel/kprobes/core.c   |   6 +-
>  arch/x86/kernel/traps.c          |   6 +-
>  arch/x86/lib/getuser.S           |  12 ++--
>  arch/x86/lib/putuser.S           |  10 ++--
>  arch/x86/mm/extable.c            | 120 ++++++++++++++++++++++++++++++++++-----
>  arch/x86/mm/fault.c              |   2 +-
>  arch/x86/xen/enlighten.c         |   4 +-
>  11 files changed, 145 insertions(+), 45 deletions(-)

I'd also like to see the use of set_fs() (which has been grossly
misnamed since ancient versions of Linux) significantly reduced.  Many
of these uses are in compat syscalls, which do:
- read the user memory
- convert it to the native format
- call set_fs(KERNEL_DS)
- pass it to the native syscall which does another user copy.

By separating the core syscall code from the userspace accesses so
that it only touches kernel memory, you can eliminate the set_fs() and
the extra copy from the compat case.

I had started work on this a while back but never finished it.  I'll
look at bringing it up to date.

--
Brian Gerst