Re: [PATCH 1/1] perf: Fix warning from concurrent read/write of perf_event_pmu_context

[Ravi Bangoria <ravi.bangoria@amd.com>]

Hi James,

On 27-Jan-23 8:01 PM, James Clark wrote:
> When running two Perf sessions, the following warning can appear:
> 
>   WARNING: CPU: 1 PID: 2245 at kernel/events/core.c:4925 put_pmu_ctx+0x1f0/0x278
>   Modules linked in: xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT nf_reject_ipv4 xt_tcpudp ip6table_mangle ip6table_nat iptable_mangle iptable_nat nf_nat nf_conntrack libcrc32c nf_defrag_ipv6 nf_defrag_ipv4 ip6table_filter ip6_tables iptable_filter bridge stp llc coresight_stm stm_core coresight_etm4x coresight_tmc coresight_replicator coresight_funnel coresight_tpiu coresight arm_spe_pmu ip_tables x_tables ipv6 xhci_pci xhci_pci_renesas r8169
>   CPU: 1 PID: 2245 Comm: perf Not tainted 6.2.0-rc4+ #1
>   pstate: 20400009 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
>   pc : put_pmu_ctx+0x1f0/0x278
>   lr : put_pmu_ctx+0x1b4/0x278
>   sp : ffff80000dfcbc20
>   x29: ffff80000dfcbca0 x28: ffff008004f00000 x27: ffff00800763a928
>   x26: ffff00800763a928 x25: 00000000000000c0 x24: 0000000000000000
>   x23: 00000000000a0003 x22: ffff00837df74088 x21: ffff80000dfcbd18
>   x20: 0000000000000000 x19: ffff00800763a6c0 x18: 0000000000000000
>   x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
>   x14: 0000000000000000 x13: ffff80000dfc8000 x12: ffff80000dfcc000
>   x11: be58ab6d2939e700 x10: be58ab6d2939e700 x9 : 0000000000000000
>   x8 : 0000000000000001 x7 : 0000000000000000 x6 : 0000000000000000
>   x5 : ffff00800093c9c0 x4 : 0000000000000000 x3 : ffff80000dfcbca0
>   x2 : ffff008004f00000 x1 : ffff8000082403c4 x0 : 0000000000000000
>   Call trace:
>    put_pmu_ctx+0x1f0/0x278
>    _free_event+0x2bc/0x3d0
>    perf_event_release_kernel+0x444/0x4bc
>    perf_release+0x20/0x30
>    __fput+0xe4/0x25c
>    ____fput+0x1c/0x28
>    task_work_run+0xc4/0xe8
>    do_notify_resume+0x10c/0x164
>    el0_svc+0xb4/0xdc
>    el0t_64_sync_handler+0x84/0xf0
>    el0t_64_sync+0x190/0x194
> 
> This is because there is no locking around the access of "if
> (!epc->ctx)" in find_get_pmu_context() and when it is set to NULL in
> put_pmu_ctx().
> 
> The decrement of the reference count in put_pmu_ctx() also happens
> outside of the spinlock, leading to the possibility of this order of
> events, and the context being cleared in put_pmu_ctx(), after its
> refcount is non zero:
> 
>  CPU0                                   CPU1
>  find_get_pmu_context()
>    if (!epc->ctx) == false
>                                         put_pmu_ctx()
>                                         atomic_dec_and_test(&epc->refcount) == true
>                                         epc->refcount == 0
>      atomic_inc(&epc->refcount);
>      epc->refcount == 1
>                                         list_del_init(&epc->pmu_ctx_entry);
> 	                                      epc->ctx = NULL;
> 
> Another issue is that WARN_ON for no active PMU events in put_pmu_ctx()
> is outside of the lock. If the perf_event_pmu_context is an embedded
> one, even after clearing it, it won't be deleted and can be re-used. So
> the warning can trigger. For this reason it also needs to be moved
> inside the lock.
> 
> The above warning is very quick to trigger on Arm by running these two
> commands at the same time:
> 
>   while true; do perf record -- ls; done
>   while true; do perf record -- ls; done

These dose not trigger WARN_ON on my x86 machine, however, the C reproducer
provided by syzbot[1] does trigger it.

[1]: https://syzkaller.appspot.com/text?tag=ReproC&x=17beacbc480000

Thanks,
Ravi