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Date: Tue, 27 Sep 2016 14:35:33 -0700
From: "LeMay, Michael" <michael.lemay@...el.com>
To: Rich Felker <dalias@...c.org>
Cc: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Re: [RFC] Support for segmentation-hardened SafeStack
On 9/27/2016 07:43, Rich Felker wrote:
> On Mon, Sep 26, 2016 at 11:05:06PM -0700, LeMay, Michael wrote:
...
>> Arguments, whether variadic or not, are still passed on the main
>> (safe) stack like usual, and they can be used in-place.
> Here I think we're just differing on what "used in-place" means. For
> me that would include the ability to take their addresses. I assume
> you're just talking about using the values.
I see your point now. Yes, when SafeStack determines that a local
variable or argument may be accessed unsafely, it moves or copies
(respectively) that allocation to the unsafe stack. Incidentally, I
thought that just taking the address of a local variable or argument
(e.g. for pointer comparisons within a single function) would not
necessarily result in it being moved to the unsafe stack, but re-reading
the SafeStack pass and running some tests showed me that the pass
currently does move such allocations to the unsafe stack.
...
>
> This is another place where I think we're just using terms
> differently. From my perspective (the formal C language) variadic
> argument handling does not involve taking or dereferencing addresses
> on the stack; those are just va_list/va_arg implementation details. At
> the level of the formal language I think there are no exceptions; in
> all cases where the address on "the stack" leaks outside the scope of
> what the compiler can see/control, "the stack" it's on has to be the
> unsafe stack.
Yes, we're in agreement. For completeness, I'll note that there are
other ways for safe stack pointers to leak:
http://clang.llvm.org/docs/SafeStack.html#known-security-limitations
Thanks,
Michael
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