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Date: Thu, 18 Apr 2024 15:19:26 -0700
From: Alan Coopersmith <alan.coopersmith@...cle.com>
To: libc-coord@...ts.openwall.com
Subject: Re: Preventing re-use of jmp_buf?
On 4/18/24 15:07, Alan Coopersmith wrote:
> On 4/18/24 10:25, Paul Eggert wrote:
>> On 2024-04-18 03:36, Florian Weimer wrote:
>>> think it's possible to meet all the preconditions for longjmp and call
>>> it multiple times after a setjmp call, on the same buffer. Therefore, I
>>> don't think this would be a conforming or just compatible change.
>>
>> Quite right.
>>
>> However, couldn't we catch many instances of the more-important case where
>> longjmp called with a jmp_buf that's no longer valid? Something involving a
>> per-thread nonce that's tweaked each time setjmp is called, and where the
>> nonce's value is stored in both the jmp_buf and in the setjmp caller's frame,
>> so that longjmp could reliably crash if the nonce is corrupted. We could do
>> this if fortification is enabled.
>
> Right - the case that originated this conversation was:
> https://gitlab.freedesktop.org/xorg/lib/libxmu/-/issues/2
>
> In the test code, I had written test cases to make sure that the functions
> in question correctly detected and reported errors. But since the error
> handlers in question exit the program, after calling a specified error
> handling/cleanup callback, I used setjmp() and longjmp() to jump from
> the callback back to the test code.
>
> The problem was there were multiple test functions, and I had called
> setjmp() in the first, but not the second, and the jump buffer was
> invalid after the first exited, but the longjmp() was still called
> from the second one, which crashed with a garbage stack.
And I should say, the real problem was that everything worked with the
invalid call on both Solaris & Linux on x86-64 platforms - the crash
was only seen on x86-32 - having a way to clear the buffer at the end
of the function would hopefully make this crash on all platforms so
the bug could be found before the package release.
--
-Alan Coopersmith- alan.coopersmith@...cle.com
Oracle Solaris Engineering - https://blogs.oracle.com/solaris
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