Patch "s390/cpum_sf: Handle CPU hotplug remove during sampling" has been added to the 5.15-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    s390/cpum_sf: Handle CPU hotplug remove during sampling

to the 5.15-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     s390-cpum_sf-handle-cpu-hotplug-remove-during-sampli.patch
and it can be found in the queue-5.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit a87f5958d1dc60a2d2bc7547d8a214d760646238
Author: Thomas Richter <tmricht@xxxxxxxxxxxxx>
Date:   Fri Oct 25 12:27:53 2024 +0200

    s390/cpum_sf: Handle CPU hotplug remove during sampling
    
    [ Upstream commit a0bd7dacbd51c632b8e2c0500b479af564afadf3 ]
    
    CPU hotplug remove handling triggers the following function
    call sequence:
    
       CPUHP_AP_PERF_S390_SF_ONLINE  --> s390_pmu_sf_offline_cpu()
       ...
       CPUHP_AP_PERF_ONLINE          --> perf_event_exit_cpu()
    
    The s390 CPUMF sampling CPU hotplug handler invokes:
    
     s390_pmu_sf_offline_cpu()
     +-->  cpusf_pmu_setup()
           +--> setup_pmc_cpu()
                +--> deallocate_buffers()
    
    This function de-allocates all sampling data buffers (SDBs) allocated
    for that CPU at event initialization. It also clears the
    PMU_F_RESERVED bit. The CPU is gone and can not be sampled.
    
    With the event still being active on the removed CPU, the CPU event
    hotplug support in kernel performance subsystem triggers the
    following function calls on the removed CPU:
    
      perf_event_exit_cpu()
      +--> perf_event_exit_cpu_context()
           +--> __perf_event_exit_context()
                +--> __perf_remove_from_context()
                     +--> event_sched_out()
                          +--> cpumsf_pmu_del()
                               +--> cpumsf_pmu_stop()
                                    +--> hw_perf_event_update()
    
    to stop and remove the event. During removal of the event, the
    sampling device driver tries to read out the remaining samples from
    the sample data buffers (SDBs). But they have already been freed
    (and may have been re-assigned). This may lead to a use after free
    situation in which case the samples are most likely invalid. In the
    best case the memory has not been reassigned and still contains
    valid data.
    
    Remedy this situation and check if the CPU is still in reserved
    state (bit PMU_F_RESERVED set). In this case the SDBs have not been
    released an contain valid data. This is always the case when
    the event is removed (and no CPU hotplug off occured).
    If the PMU_F_RESERVED bit is not set, the SDB buffers are gone.
    
    Signed-off-by: Thomas Richter <tmricht@xxxxxxxxxxxxx>
    Reviewed-by: Hendrik Brueckner <brueckner@xxxxxxxxxxxxx>
    Signed-off-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/s390/kernel/perf_cpum_sf.c b/arch/s390/kernel/perf_cpum_sf.c
index a8ba3887b367f..22893bf86f650 100644
--- a/arch/s390/kernel/perf_cpum_sf.c
+++ b/arch/s390/kernel/perf_cpum_sf.c
@@ -1896,7 +1896,9 @@ static void cpumsf_pmu_stop(struct perf_event *event, int flags)
 	event->hw.state |= PERF_HES_STOPPED;
 
 	if ((flags & PERF_EF_UPDATE) && !(event->hw.state & PERF_HES_UPTODATE)) {
-		hw_perf_event_update(event, 1);
+		/* CPU hotplug off removes SDBs. No samples to extract. */
+		if (cpuhw->flags & PMU_F_RESERVED)
+			hw_perf_event_update(event, 1);
 		event->hw.state |= PERF_HES_UPTODATE;
 	}
 	perf_pmu_enable(event->pmu);