Re: Prevent inconsistent CPU state after sequence of dlclose/dlopen

[Mathieu Desnoyers]

On 2025-01-10 12:46, Florian Weimer wrote:
> * 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.
>
> Oh, so CPU 1 initially executes some code, returns to some safe,
> persistent code (“the execute unrelated code” part), this code
> synchronizes with the dlclose and the dlopen that execute on CPU 0,
> obtains a pointer to some supposedly safely published function in the
> newly mapped object, and calls it.  And that fails because previously
> cached information about the code is invalid?

Correct.

> Additional awkwardness may result if the initial execution is
> speculative, and the code on CPU 1 only synchronizes with the dlopen,
> and not the previous dlclose because it does not know about it at all?

I'm not sure I follow this last example. Can you explain further what
you have in mind ?

Thanks,

Mathieu

> Thanks,
> Florian

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