Re: 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