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

[Johannes Berg]

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.

johannes