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From: "Philippe Mathieu-Daudé" <philmd@redhat.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Kevin Wolf" <kwolf@redhat.com>,
	"Tobin Feldman-Fitzthum" <tobin@ibm.com>,
	qemu-devel@nongnu.org, "Stefan Hajnoczi" <stefanha@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Alex Bennée" <alex.bennee@linaro.org>,
	"Daniele Buono" <dbuono@linux.vnet.ibm.com>
Subject: Re: [PATCH 0/4] Add support for SafeStack
Date: Tue, 5 May 2020 15:56:00 +0200	[thread overview]
Message-ID: <c6ac2d0e-34bf-9927-f2fe-2ef0408dcbfb@redhat.com> (raw)
In-Reply-To: <20200505133111.GM764268@redhat.com>

On 5/5/20 3:31 PM, Daniel P. Berrangé wrote:
> On Tue, May 05, 2020 at 03:15:18PM +0200, Philippe Mathieu-Daudé wrote:
>> +Alex & Daniel who keep track on CI stuff.
>>
>> On 4/29/20 9:44 PM, Daniele Buono wrote:
>>> LLVM supports SafeStack instrumentation to protect against stack buffer
>>> overflows, since version 3.7
>>>
>>>   From https://clang.llvm.org/docs/SafeStack.html:
>>> "It works by separating the program stack into two distinct regions: the
>>> safe stack and the unsafe stack. The safe stack stores return addresses,
>>> register spills, and local variables that are always accessed in a safe
>>> way, while the unsafe stack stores everything else. This separation
>>> ensures that buffer overflows on the unsafe stack cannot be used to
>>> overwrite anything on the safe stack."
>>>
>>> Unfortunately, the use of two stack regions does not cope well with
>>> QEMU's coroutines. The second stack region is not properly set up with
>>> both ucontext and sigaltstack, so multiple coroutines end up sharing the
>>> same memory area for the unsafe stack, causing undefined behaviors at
>>> runtime (and most iochecks to fail).
>>>
>>> This patch series fixes the implementation of the ucontext backend and
>>> make sure that sigaltstack is never used if the compiler is applying
>>> the SafeStack instrumentation. It also adds a configure flag to enable
>>> SafeStack, and enables iotests when SafeStack is used.
>>>
>>> This is an RFC mainly because of the low-level use of the SafeStack
>>> runtime.
>>> When running swapcontext(), we have to manually set the unsafe stack
>>> pointer to the new area allocated for the coroutine. LLVM does not allow
>>> this by using builtin, so we have to use implementation details that may
>>> change in the future.
>>> This patch has been tested briefly ( make check on an x86 system ) with
>>> clang v3.9, v4.0, v5.0, v6.0
>>> Heavier testing, with make check-acceptance has been performed with
>>> clang v7.0
>>
>> I noticed building using SafeStack is slower, and running with it is even
>> sloooower. It makes sense to have this integrated if we use it regularly. Do
>> you have plan for this? Using public CI doesn't seem reasonable.
> 
> The runtime behaviour is rather odd, given the docs they provide:
> 
> "The performance overhead of the SafeStack instrumentation is
>   less than 0.1% on average across a variety of benchmarks
>   This is mainly because most small functions do not have any
>   variables that require the unsafe stack and, hence, do not
>   need unsafe stack frames to be created. The cost of creating
>   unsafe stack frames for large functions is amortized by the
>   cost of executing the function.
> 
>    In some cases, SafeStack actually improves the performance"

I'm sorry I was testing this with a single core instead of all of 
them... Thanks for looking at the doc.

> 
> Regards,
> Daniel
> 



  reply	other threads:[~2020-05-05 13:57 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-29 19:44 [PATCH 0/4] Add support for SafeStack Daniele Buono
2020-04-29 19:44 ` [PATCH 1/4] coroutine: support SafeStack in ucontext backend Daniele Buono
2020-05-21  9:44   ` Stefan Hajnoczi
2020-05-22 15:18     ` Daniele Buono
2020-05-27 10:34       ` Stefan Hajnoczi
2020-04-29 19:44 ` [PATCH 2/4] coroutine: Add check for SafeStack in sigalstack Daniele Buono
2020-05-04 14:56   ` Philippe Mathieu-Daudé
2020-05-21  9:49   ` Stefan Hajnoczi
2020-05-27 17:56     ` Daniele Buono
2020-04-29 19:44 ` [PATCH 3/4] configure: add flag to enable SafeStack Daniele Buono
2020-05-21  9:52   ` Stefan Hajnoczi
2020-05-22 15:24     ` Daniele Buono
2020-05-27 11:12       ` Stefan Hajnoczi
2020-05-27 13:48         ` Daniele Buono
2020-04-29 19:44 ` [PATCH 4/4] check-block: Enable iotests with SafeStack Daniele Buono
2020-05-21  9:59   ` Stefan Hajnoczi
2020-05-22 15:35     ` Daniele Buono
2020-05-27 11:13       ` Stefan Hajnoczi
2020-05-04 14:55 ` [PATCH 0/4] Add support for SafeStack Philippe Mathieu-Daudé
2020-05-05 13:15 ` Philippe Mathieu-Daudé
2020-05-05 13:31   ` Daniel P. Berrangé
2020-05-05 13:56     ` Philippe Mathieu-Daudé [this message]
2020-05-13 14:48       ` Daniele Buono
2020-05-21 10:00         ` Stefan Hajnoczi

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