Re: [PATCH] livepatch: Fix races between stack check and module load/unload

[Miroslav Benes <mbenes@suse.cz>]

Hi Andrea,

Cc+ Petr Pavlu (you can find the whole thread at 
https://lore.kernel.org/live-patching/20261012150758.207983-1-parri.andrea@gmail.com/T/#t)

On Mon, 12 Oct 2026, Andrea Parri wrote:

> Unloading a patched module while a livepatch transition is in progress
> can crash the kernel in klp_find_ops(), called from the scheduler.
> KCSAN reports
> 
>   BUG: KCSAN: data-race in klp_find_ops / klp_unpatch_func
> 
>   write (marked) to 0xffff9094c09ece10 of 8 bytes by task 381 on cpu 2:
>    klp_unpatch_func+0x1bd/0x280
>    klp_unpatch_object+0x6a/0x90
>    klp_cleanup_module_patches_limited.cold+0x4a/0xd4
>    klp_module_going+0x54/0x80
>    __do_sys_delete_module+0x247/0x380
>    __x64_sys_delete_module+0x39/0x50
>    ...
> 
>   read to 0xffff9094c09ece10 of 8 bytes by task 146 on cpu 1:
>    klp_find_ops+0x55/0x90
>    klp_check_and_switch_task+0x228/0x320
>    klp_try_switch_task+0xb4/0xc0
>    __klp_sched_try_switch+0x9a/0xb0
>    __schedule+0x60a/0x1450
>    schedule+0x3b/0x100
>    schedule_timeout+0xc0/0x160
>    ra_target_a_loop+0x97/0xa0 [klp_race_target_a]
>    ...
> 
> followed by
> 
>   Oops: general protection fault, probably for non-canonical address
>   0xdead000000000110: 0000 [#2] SMP NOPTI
>   RIP: 0010:klp_find_ops+0x55/0x90
>    ...
>    klp_check_and_switch_task+0x228/0x320
>    klp_try_switch_task+0xb4/0xc0
>    __klp_sched_try_switch+0x9a/0xb0
>    __schedule+0x60a/0x1450

Interesting. So to sum it up... you load a live patch which contains a fix 
for a module and while the transition is on, you remove the module and it 
crashes (btw, I do not think that those traces are necessary here. More 
btw, it would be nice to have a human readable changelog instead of what 
looks like AI generated description of the patch itself).

How did you find the issue? Is it a real problem in production?

I wonder whether we could just decline delete_module() in this case and 
return -EBUSY. It should be a rare operation anyway. Alternatively we 
could increase refcounts of all patched modules during a transition and 
decrease them after. Petr P., what do you think?

I am not saying we should not do the RCU protection but it feels like a 
can of worms a bit so if there is an easier way...

Regards
Miroslav