[PATCH 3/5] perf: Add pmu get/put

[Peter Zijlstra]

On Tue, Oct 08, 2024 at 01:34:59PM -0500, Lucas De Marchi wrote:
> If a pmu is unregistered while there's an active event, perf will still
> access the pmu via event->pmu, even after the event is destroyed. This
> makes it difficult for drivers like i915 that can be unbound from the
> HW.
> 
> 	BUG: KASAN: use-after-free in exclusive_event_destroy+0xd8/0xf0
> 	Read of size 4 at addr ffff88816e2bb63c by task perf/7748
> 
> i915 tries to cope with it by installing a event->destroy, but that is
> not sufficient: if pmu is released by the driver, it will still crash
> since event->pmu is still used.
> 
> Moreover, even with that use-after-free fixed by adjusting the order in
> _free_event() or delaying the free by the driver, kernel still oops when
> closing the event fd related to a unregistered pmu: the percpu variables
> allocated on perf_pmu_register() would already be released. One such
> crash is:
> 
> 	BUG: KASAN: user-memory-access in _raw_spin_lock_irqsave+0x88/0x100
> 	Write of size 4 at addr 00000000ffffffff by task perf/727
> 
> 	CPU: 0 UID: 0 PID: 727 Comm: perf Not tainted 6.12.0-rc1-DEMARCHI-dxnf+ #9
> 	Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS unknown 2/2/2022
> 	Call Trace:
> 	 <TASK>
> 	 dump_stack_lvl+0x5f/0x90
> 	 print_report+0x4d3/0x50a
> 	 ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> 	 ? kasan_addr_to_slab+0xd/0xb0
> 	 kasan_report+0xe2/0x170
> 	 ? _raw_spin_lock_irqsave+0x88/0x100
> 	 ? _raw_spin_lock_irqsave+0x88/0x100
> 	 kasan_check_range+0x125/0x230
> 	 __kasan_check_write+0x14/0x30
> 	 _raw_spin_lock_irqsave+0x88/0x100
> 	 ? __pfx__raw_spin_lock_irqsave+0x10/0x10
> 	 _atomic_dec_and_raw_lock_irqsave+0x89/0x110
> 	 ? __kasan_check_write+0x14/0x30
> 	 put_pmu_ctx+0x98/0x330
> 
> The fix here is to provide a set of get/put hooks that drivers can
> implement to piggy back the perf's pmu lifecycle to the driver's
> instance lifecycle.  With this, perf_pmu_unregister() can be called by
> the driver, which is then responsible for freeing the resources.

I'm confused.. probably because I still don't have any clue about
drivers and the above isn't really telling me much either.

I don't see how you get rid of the try_module_get() we do per event;
without that you can't unload the module.

And I don't see how you think it is safe to free a pmu while there are
still events around.

Nor do I really see what these new get/put methods do. I see you call
->put() where we do module_put(), and ->get() near try_module_get(), but
how is that helping?