Re: [PATCH v2 0/4] Add support for SafeStack

[Stefan Hajnoczi]

On Fri, May 29, 2020 at 04:51:18PM -0400, 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.
> 
> Changes since v1:
>  - CONFIG_SAFESTACK is now set up in configure, and not in the code
>  - Added option for a --disable-safe-stack in configure
>  - Configure checks if SafeStack is enabled by default in the compiler,
>    and set the CONFIG_SAFESTACK accordingly
>  - Updated some comments in the code and the commit log
> 
> NOTE: I kept configure as Patch #3. 
> The reason is that the code changes will not be enabled without the
> configure, making the code fully functional if only Patches #1 or #2 are
> applied.
> On the other hand, the configure patch will produce incorrect code if we
> request SafeStack and the other patches are not applied.
> 
> Daniele Buono (4):
>   coroutine: support SafeStack in ucontext backend
>   coroutine: add check for SafeStack in sigaltstack
>   configure: add flags to support SafeStack
>   check-block: enable iotests with SafeStack
> 
>  configure                    | 73 ++++++++++++++++++++++++++++++++++++
>  include/qemu/coroutine_int.h |  5 +++
>  tests/check-block.sh         | 12 +++++-
>  util/coroutine-sigaltstack.c |  4 ++
>  util/coroutine-ucontext.c    | 26 +++++++++++++
>  5 files changed, 119 insertions(+), 1 deletion(-)
> 
> -- 
> 2.26.2
> 
> 

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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