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

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Message-ID: <ca334a7a-12c9-4811-882b-988d98c0eb14@efficios.com>
Date: Fri, 10 Jan 2025 12:13:58 -0500
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: Florian Weimer <fweimer@...hat.com>
Cc: libc-alpha <libc-alpha@...rceware.org>,
 "carlos@...hat.com" <carlos@...hat.com>, Mark Rutland
 <mark.rutland@....com>, Peter Zijlstra <peterz@...radead.org>,
 linux-kernel <linux-kernel@...r.kernel.org>, x86@...nel.org,
 paulmck <paulmck@...nel.org>, Michael Jeanson <mjeanson@...icios.com>
Subject: Re: Prevent inconsistent CPU state after sequence of dlclose/dlopen

On 2025-01-10 12:04, Florian Weimer wrote:
> * Mathieu Desnoyers:
> 
>> 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.
>>
>> I maintain the membarrier(2) system call, which provides a
>> MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE command for this
>> purpose. It's been there since Linux 4.16. It can be configured
>> out (CONFIG_MEMBARRIER=n), but it's enabled by default.
>>
>> Calling this after dlclose() in glibc would prevent this issue.
>>
>> Is it handled in some other way, or should we open a bugzilla
>> entry to track this ?
> 
> There is nothing special about dlopen/dlclose, we just use mmap/munmap.
> If there is a synchronization problem, we'd have to add to add barriers
> to mmap and munmap.
> 
> But why isn't it up to the kernel to handle this correctly?

As I mentioned to Peter, we could add this barrier within mprotect(2)
and munmap(2) in the following cases:

- mprotect removes PROT_EXEC from a mapping,
- munmap unmaps a PROT_EXEC mapping.

We could even go further and batch this: we only need to
issue membarrier-sync-core on the following sequence for an mm:

On either of those, set current->mm->pending_membarrier_sync_core = true:
   - mprotect removes PROT_EXEC from a mapping, or
   - munmap unmaps a PROT_EXEC mapping,

And then, if current->mm->pending_membarrier_sync_core == true when:
   - mmap is called to create a PROT_EXEC mapping, or
   - mprotect sets PROT_EXEC on a mapping.

   invoke membarrier sync-core and set
   current->mm_pending_membarrier = false

Thoughts ?

Thanks,

Mathieu

> 
> Thanks,
> Florian
> 

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