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Re: [PATCH 0/4] Add support for SafeStack


From: Daniele Buono
Subject: Re: [PATCH 0/4] Add support for SafeStack
Date: Wed, 13 May 2020 10:48:04 -0400
User-agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:68.0) Gecko/20100101 Thunderbird/68.8.0

Hello everybody, just pinging since it it's been a few days.

On 5/5/2020 9:56 AM, Philippe Mathieu-Daudé wrote:
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






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