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Their call traces are like this: >> >> [346278.191038] __schedule+0x2d8/0x890 >> [346278.191046] schedule+0x4e/0xb0 >> [346278.191049] perf_event_free_task+0x220/0x270 >> [346278.191056] ? init_wait_var_entry+0x50/0x50 >> [346278.191060] copy_process+0x663/0x18d0 >> [346278.191068] kernel_clone+0x9d/0x3d0 >> [346278.191072] __do_sys_clone+0x5d/0x80 >> [346278.191076] __x64_sys_clone+0x25/0x30 >> [346278.191079] do_syscall_64+0x5c/0xc0 >> [346278.191083] ? syscall_exit_to_user_mode+0x27/0x50 >> [346278.191086] ? do_syscall_64+0x69/0xc0 >> [346278.191088] ? irqentry_exit_to_user_mode+0x9/0x20 >> [346278.191092] ? irqentry_exit+0x19/0x30 >> [346278.191095] ? exc_page_fault+0x89/0x160 >> [346278.191097] ? asm_exc_page_fault+0x8/0x30 >> [346278.191102] entry_SYSCALL_64_after_hwframe+0x44/0xae >> >> The task was waiting for the refcount become to 1, but from the vmcore, >> we found the refcount has already been 1. It seems that the task didn't >> get woken up by perf_event_release_kernel() and got stuck forever. The >> below scenario may cause the problem. >> >> Thread A Thread B >> ... ... >> perf_event_free_task perf_event_release_kernel >> ... >> acquire event->child_mutex >> ... >> get_ctx >> ... release event->child_mutex >> acquire ctx->mutex >> ... >> perf_free_event (acquire/release event->child_mutex) >> ... >> release ctx->mutex >> wait_var_event >> acquire ctx->mutex >> acquire event->child_mutex >> # move existing events to free_list >> release event->child_mutex >> release ctx->mutex >> put_ctx >> ... ... >> >> In this case, all events of the ctx have been freed, so we couldn't >> find the ctx in free_list and Thread A will miss the wakeup. It's thus >> necessary to add a wakeup after dropping the reference. >> >> Fixes: 1cf8dfe8a661 ("perf/core: Fix race between close() and fork()") >> Signed-off-by: Haifeng Xu >> Reviewed-by: Frederic Weisbecker > > FWIW, this looks good to me, but I haven't yet been able to write a test to > exercise this: perf_event_free_task() is only called if > perf_event_init_context() fails or of copy_process() fails partway through, and > while it should be possible to make the latter fail consistently by messing > with cgroups, I haven't had the time to work all that out. > Hi, Mark. This problem seems similar to this bug reported by syzbot. https://lore.kernel.org/all/00000000000057102e058e722bba@google.com/T/#mbb1d50748ff3190738a9754bdff118e640fbb3a3 > So I think there's a reliable DoS here, but I haven't had the time to go write > that myself. It would be nice if we actually had a test for this. > > I reckon that in addition to the Fixes tag, this needs: > > Cc: stable@vger.kernel.org > Ok, I'll add this tag next version. >> --- >> Changes since v1: >> - Add the fixed tag. >> - Simplify v1's patch. (Frederic) >> >> Changes since v2: >> - Use Reviewed-by tag instead of Signed-off-by tag. >> --- >> kernel/events/core.c | 13 +++++++++++++ >> 1 file changed, 13 insertions(+) >> >> diff --git a/kernel/events/core.c b/kernel/events/core.c >> index 4f0c45ab8d7d..15c35070db6a 100644 >> --- a/kernel/events/core.c >> +++ b/kernel/events/core.c >> @@ -5340,6 +5340,7 @@ int perf_event_release_kernel(struct perf_event *event) >> again: >> mutex_lock(&event->child_mutex); >> list_for_each_entry(child, &event->child_list, child_list) { >> + void *var = NULL; >> >> /* >> * Cannot change, child events are not migrated, see the >> @@ -5380,11 +5381,23 @@ int perf_event_release_kernel(struct perf_event *event) >> * this can't be the last reference. >> */ >> put_event(event); >> + } else { >> + var = &ctx->refcount; >> } >> >> mutex_unlock(&event->child_mutex); >> mutex_unlock(&ctx->mutex); >> put_ctx(ctx); >> + >> + if (var) { >> + /* >> + * If perf_event_free_task() has deleted all events from the >> + * ctx while the child_mutex got released above, make sure to >> + * notify about the preceding put_ctx(). >> + */ >> + smp_mb(); /* pairs with wait_var_event() */ >> + wake_up_var(var); >> + } >> goto again; >> } >> mutex_unlock(&event->child_mutex); > > I was a bit worrited that we're doing the wakeup with the event->child_mutex > held; Actually the event->child_mutex has been released before doing the wakeup. AFAICT that looks to be safe, but I'm not a scheduler expert. > > FWIW: > > Acked-by: Mark Rutland > > Mark. Thanks! > >> -- >> 2.25.1 >>