Re: [PATCH] stack-protector: Check stack canary for noreturn function

[Jeff Law <jeffreyalaw@gmail.com>]

On 8/2/2022 11:43 AM, H.J. Lu wrote:
> On Sat, Jul 30, 2022 at 1:30 PM Jeff Law via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
>>
>>
>> On 7/14/2022 3:55 PM, H.J. Lu via Gcc-patches wrote:
>>> Check stack canary for noreturn function to catch stack corruption
>>> before calling noreturn function.  For C++, check stack canary when
>>> throwing exception or resuming stack unwind to avoid corrupted stack.
>>>
>>> gcc/
>>>
>>>        PR middle-end/58245
>>>        * calls.cc (expand_call): Check stack canary for noreturn
>>>        function.
>>>
>>> gcc/testsuite/
>>>
>>>        PR middle-end/58245
>>>        * c-c++-common/pr58245-1.c: New test.
>>>        * g++.dg/pr58245-1.C: Likewise.
>>>        * g++.dg/fstack-protector-strong.C: Adjusted.
>> But is this really something we want?   I'd actually lean towards
>> eliminating the useless load -- I don't necessarily think we should be
>> treating non-returning paths specially here.
>>
>> The whole point of the stack protector is to prevent the *return* path
>> from going to an attacker controlled location.  I'm not sure checking
>> the protector at this point actually does anything particularly useful.
> throw is marked no return.   Since the unwind library may read
> the stack contents to unwind stack, it the stack is corrupted, the
> exception handling may go wrong.   Should we handle this case?
That's the question I think we need to answer.  The EH paths are a known 
security issue on Windows and while ours are notably different I'm not 
sure if there's a real attack surface in those paths.  My sense is that 
if we need to tackle this that doing so on the throw side might be 
better as it's closer conceptually to when//how we check the canary for 
a normal return.

jeff
>
>   --
> H.J.