linux-kernel - Re: [PATCH v5 0/4] x86: modify_ldt improvement, test, and config option

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Date:	Thu, 30 Jul 2015 11:53:34 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>
Subject: Re: [PATCH v5 0/4] x86: modify_ldt improvement, test, and config
 option

On 07/28/2015 01:29 AM, Andy Lutomirski wrote:
> This is intended for x86/urgent.  Sorry for taking so long, but it
> seemed nice to avoid breaking Xen.
>
> This fixes the "dazed and confused" issue which was exposed by the
> CVE-2015-5157 fix.  It's also probably a good general attack surface
> reduction, and it replaces some scary code with IMO less scary code.
>
> Also, servers and embedded systems should probably turn off modify_ldt.
> This makes that possible.
>
> Boris, could I get a Tested-by, assuming this works for you?

As far as Xen guests are concerned,

Tested-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>

But ldt_gdt_32 test segfaults on 64-bit kernels. Baremetal and virt. I 
thought it worked for me before but can't reproduce this with older 
patches. Does it work for you?


-boris


>
> Willy and Kees: I left the config option alone.  The -tiny people will
> like it, and we can always add a sysctl of some sort later.
>
> Changes from v4:
>   - Fix Xen even better (patch 1 is new).
>   - Reorder the patches to make a little more sense.
>
> Changes from v3:
>   - Hopefully fixed Xen.
>   - Fixed 32-bit test case on 32-bit native kernel.
>   - Fix bogus vumnap for some LDT sizes.
>   - Strengthen test case to check all LDT sizes (catches bogus vunmap).
>   - Lots of cleanups, mostly from Borislav.
>   - Simplify IPI code using on_each_cpu_mask.
>
> Changes from v2:
>   - Allocate ldt_struct and the LDT entries separately.  This should fix Xen.
>   - Stop using write_ldt_entry, since I'm pretty sure it's unnecessary now
>     that we no longer mutate an in-use LDT.  (Xen people, can you check?)
>
> Changes from v1:
>   - The config option is new.
>   - The test case is new.
>   - Fixed a missing allocation failure check.
>   - Fixed a use-after-free on fork().
>
> Andy Lutomirski (4):
>    x86/xen: Unmap aliases in xen_alloc_ldt and xen_free_ldt
>    x86/ldt: Make modify_ldt synchronous
>    selftests/x86, x86/ldt: Add a selftest for modify_ldt
>    x86/ldt: Make modify_ldt optional
>
>   arch/x86/Kconfig                      |  17 ++
>   arch/x86/include/asm/desc.h           |  15 -
>   arch/x86/include/asm/mmu.h            |   5 +-
>   arch/x86/include/asm/mmu_context.h    |  68 ++++-
>   arch/x86/kernel/Makefile              |   3 +-
>   arch/x86/kernel/cpu/common.c          |   4 +-
>   arch/x86/kernel/cpu/perf_event.c      |  16 +-
>   arch/x86/kernel/ldt.c                 | 262 +++++++++--------
>   arch/x86/kernel/process_64.c          |   6 +-
>   arch/x86/kernel/step.c                |   8 +-
>   arch/x86/power/cpu.c                  |   3 +-
>   arch/x86/xen/enlighten.c              |  12 +
>   kernel/sys_ni.c                       |   1 +
>   tools/testing/selftests/x86/Makefile  |   2 +-
>   tools/testing/selftests/x86/ldt_gdt.c | 520 ++++++++++++++++++++++++++++++++++
>   15 files changed, 787 insertions(+), 155 deletions(-)
>   create mode 100644 tools/testing/selftests/x86/ldt_gdt.c
>

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