[PATCH 3/5] um: Do a double clone to disable rseq

[Tiwei Bie]

On 5/28/24 7:57 PM, Johannes Berg wrote:
> On Tue, 2024-05-28 at 18:16 +0800, Tiwei Bie wrote:
>> On 5/28/24 4:54 PM, benjamin at sipsolutions.net wrote:
>>> From: Benjamin Berg <benjamin.berg at intel.com>
>>>
>>> Newer glibc versions are enabling rseq support by default. This remains
>>> enabled in the cloned child process, potentially causing the host kernel
>>> to write/read memory in the child.
>>>
>>> It appears that this was purely not an issue because the used memory
>>> area happened to be above TASK_SIZE and remains mapped.
>>
>> I also encountered this issue. In my case, with "Force a static link"
>> (CONFIG_STATIC_LINK) enabled, UML will crash immediately every time
>> it starts up. I worked around this by setting the glibc.pthread.rseq
>> tunable via GLIBC_TUNABLES [1] before launching UML.
>>
>> So another easy way to work around this issue without introducing runtime
>> overhead might be to add the GLIBC_TUNABLES=glibc.pthread.rseq=0 environment
>> variable and exec /proc/self/exe in UML on startup.
>>
> 
> It's also a bit of a question what to rely on - this would introduce a
> dependency on glibc behaviour, whereas doing the double-clone proposed
> here will work purely because of host kernel behaviour, regardless of
> what part of the system set up rseq, how the tunables work, etc.

Makes sense. My previous concern was primarily about the runtime overhead,
but after taking a closer look at the patch, I realized that the double-clone
won't happen on the critical path, so there shouldn't be any performance
issues. I also think the double-clone proposal is better. :)

Regards,
Tiwei