Prevent inconsistent CPU state after sequence of dlclose/dlopen

[Mathieu Desnoyers]

On 2025-01-10 12:10, 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.

That's not the scenario though. In this scenario, CPU 1 executes
_unrelated code_ while we unmap @addr.

The issue is the stale CPU state that persists.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com