Prevent inconsistent CPU state after sequence of dlclose/dlopen

[Adhemerval Zanella Netto]

On 10/01/25 14:10, Florian Weimer wrote:
> * Mathieu Desnoyers:
> 
>> On 2025-01-10 11:54, Peter Zijlstra wrote:
>>> On Fri, Jan 10, 2025 at 10:55:36AM -0500, Mathieu Desnoyers wrote:
>>>> Hi,
>>>>
>>>> I was discussing with Mark Rutland recently, and he pointed out that a
>>>> sequence of dlclose/dlopen mapping new code at the same addresses in
>>>> multithreaded environments is an issue on ARM, and possibly on Intel/AMD
>>>> with the newer TLB broadcast maintenance.
>>> What is the exact race? Should not munmap() invalidate the TLBs
>>> before
>>> it allows overlapping mmap() to complete?
>>
>> The race Mark mentioned (on ARM) is AFAIU the following scenario:
>>
>> CPU 0                     CPU 1
>>
>> - dlopen()
>>   - mmap PROT_EXEC @addr
>>                           - fetch insn @addr, CPU state expects unchanged insn.
>>                           - execute unrelated code
>> - dlclose(addr)
>>   - munmap @addr
>> - dlopen()
>>   - mmap PROT_EXEC @addr
>>                           - fetch new insn @addr. Incoherent CPU state.
> 
> Unmapping an object while code is executing in it is undefined.
> 
> We have a problem with things like pthread_atfork handlers.  We can't
> use locking there because fork handlers are expected to perform ample
> locking themselves, and an extra lock around them would run into lock
> ordering issues.  (We tried for unrelated reasons and saw deadlocks in
> applications.)
> 
> What we can do is bump a reference counter while we run a pthread_atfork
> callback (we already associate them with DSOs) and skip the munmap part
> in dlclose if the counter is not zero.  We can complete the unmapping
> after the fork handler returns (maybe in the parent only).

We can also make dlclose a no-op (like some runtimes do), although this
has other implications.

> 
> There might be other callbacks besides fork handlers that have this
> problem.  A similar treatment is possible for some of them, hopefully
> all of them in glibc.  We cannot cover things like std::shared_ptr
> destructor calls, though.  But adding more barriers won't fix those,
> either.
> 
> Thanks,
> Florian
>