From fa091f46c3833fb22384f10eade2b4e1e1d0b278 Mon Sep 17 00:00:00 2001 From: Kyle Zeng Date: Thu, 6 Aug 2026 13:56:55 -0700 Subject: [PATCH 1/2] perf: Reject exited events as group leaders perf_event_remove_on_exec() sets remove-on-exec events to the EXIT state and detaches their group relationships. The event's file descriptor can remain open, however, and perf_event_open() currently accepts that event as a group leader because its early validation rejects only REVOKED and DEAD events. A new sibling can consequently be linked to the detached leader. When the leader is closed, perf_group_detach() observes that its PERF_ATTACH_GROUP bit is already clear and skips the new sibling. The sibling then retains a group_leader pointer to the freed event. Reject group leaders in the EXIT state. Perform the check while holding the shared context mutex so that an exec in the target task cannot detach the leader between validation and group attachment. [peterz: make the earlier test fully consistent] Fixes: 037a3c43edfb ("perf/core: Detach event groups during remove_on_exec") Assisted-by: Codex:gpt-5.6-sol Signed-off-by: Kyle Zeng Signed-off-by: Peter Zijlstra (Intel) Link: https://patch.msgid.link/20260806205655.75722-1-kylebot@openai.com --- kernel/events/core.c | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index ba5bd6a78fe7..bd25e1998faf 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -13972,7 +13972,7 @@ SYSCALL_DEFINE5(perf_event_open, goto err_fd; } group_leader = fd_file(group)->private_data; - if (group_leader->state <= PERF_EVENT_STATE_REVOKED) { + if (group_leader->state <= PERF_EVENT_STATE_EXIT) { err = -ENODEV; goto err_fd; } @@ -14103,6 +14103,12 @@ SYSCALL_DEFINE5(perf_event_open, if (group_leader->ctx != ctx) goto err_locked; + /* Recheck under ctx::mutex to serialize against remove-on-exec. */ + if (group_leader->state <= PERF_EVENT_STATE_EXIT) { + err = -ENODEV; + goto err_locked; + } + /* * Only a group leader can be exclusive or pinned */ From 42c5ca1f0a288a52878bd72a5595b08261057438 Mon Sep 17 00:00:00 2001 From: Aditya Chillara Date: Fri, 7 Aug 2026 18:11:52 +0530 Subject: [PATCH 2/2] perf/core: Fix group leader use-after-free after sibling detach perf_group_detach() handles leader and sibling detach differently. When the group leader is detached, all siblings are promoted to singleton events and their group_leader pointer is reset to themselves. When a sibling is detached, it is removed from the leader's sibling_list, but its group_leader pointer is left pointing at the old leader. That is harmless when the sibling is being closed and freed immediately, as in the DETACH_DEAD path. It is not safe when the sibling is detached but kept alive, such as during CPU hotplug with DETACH_GROUP. In that case the sibling is removed from the context, while its file descriptor can still keep it alive. A typical failing sequence is: - A group contains leader L and sibling S. - CPU hot-unplug detaches S with DETACH_GROUP, removing it from L->sibling_list but leaving S->group_leader == L. - L is later closed and freed. - A PERF_IOC_FLAG_GROUP ioctl on S follows S->group_leader and dereferences the freed leader. This was reproduced by running the perf event fuzzer, CPU hotplug, and a stress workload concurrently: Unable to handle kernel paging request at virtual address 006b6b6b6b6b6cdb CPU: 2 PID: 12489 Comm: perf_fuzzer 6.18.7 PREEMPT pc : perf_ioctl+0x34c/0xc68 x20: ffffff89a3fa2c70 x8 : 6b6b6b6b6b6b6b6b Code: 943c4a0e 340047a0 f9404a94 f9411e88 (f940b908) Call trace: perf_ioctl+0x34c/0xc68 (P) __arm64_sys_ioctl+0xa0/0xf4 invoke_syscall+0x58/0xe4 el0_svc_common+0xa8/0xdc do_el0_svc+0x1c/0x28 el0_svc+0x40/0xc0 el0t_64_sync_handler+0x68/0xdc el0t_64_sync+0x1c4/0x1c8 The fault happened in perf_ioctl(), where perf_event_for_each() follows the stale group_leader pointer and perf_event_for_each_child() then dereferences the freed leader's context. Fix the use-after-free by promoting the detached sibling to a singleton. Also fix __event_disable() cgroup accounting and event state change. Fixes: 8a49542c0554 ("perf_events: Fix races in group composition") Assisted-by: PatchWise:gpt-5.5 Signed-off-by: Aditya Chillara Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Dapeng Mi Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260807-fix-group-leader-uaf-v3-1-b0c2310c9a0d@oss.qualcomm.com --- kernel/events/core.c | 67 ++++++++++++++++++++++++-------------------- 1 file changed, 37 insertions(+), 30 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index bd25e1998faf..1a73ba0747df 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -2343,6 +2343,34 @@ static inline struct list_head *get_event_list(struct perf_event *event) &event->pmu_ctx->flexible_active; } +/* @sibling must already be unlinked from its old leader's sibling_list. */ +static void perf_promote_sibling_to_leader(struct perf_event *sibling, + struct perf_event_context *ctx, + int group_caps) +{ + /* + * Events that have PERF_EV_CAP_SIBLING require being part of + * a group and cannot exist on their own, schedule them out + * and move them into the ERROR state. Also see + * _perf_event_enable(), it will not be able to recover this + * ERROR state. + */ + if (sibling->event_caps & PERF_EV_CAP_SIBLING) + __event_disable(sibling, ctx, PERF_EVENT_STATE_ERROR); + + sibling->group_leader = sibling; + sibling->group_caps = group_caps; + + if (sibling->attach_state & PERF_ATTACH_CONTEXT) { + add_event_to_groups(sibling, ctx); + + if (sibling->state == PERF_EVENT_STATE_ACTIVE) + list_add_tail(&sibling->active_list, get_event_list(sibling)); + } + + perf_event__header_size(sibling); +} + static void perf_group_detach(struct perf_event *event) { struct perf_event *leader = event->group_leader; @@ -2366,8 +2394,9 @@ static void perf_group_detach(struct perf_event *event) */ if (leader != event) { list_del_init(&event->sibling_list); - event->group_leader->nr_siblings--; - event->group_leader->group_generation++; + leader->nr_siblings--; + leader->group_generation++; + perf_promote_sibling_to_leader(event, ctx, event->event_caps); goto out; } @@ -2377,32 +2406,14 @@ static void perf_group_detach(struct perf_event *event) * to whatever list we are on. */ list_for_each_entry_safe(sibling, tmp, &event->sibling_list, sibling_list) { - - /* - * Events that have PERF_EV_CAP_SIBLING require being part of - * a group and cannot exist on their own, schedule them out - * and move them into the ERROR state. Also see - * _perf_event_enable(), it will not be able to recover this - * ERROR state. - */ - if (sibling->event_caps & PERF_EV_CAP_SIBLING) - __event_disable(sibling, ctx, PERF_EVENT_STATE_ERROR); - - sibling->group_leader = sibling; list_del_init(&sibling->sibling_list); /* Inherit group flags from the previous leader */ - sibling->group_caps = event->group_caps; - - if (sibling->attach_state & PERF_ATTACH_CONTEXT) { - add_event_to_groups(sibling, event->ctx); - - if (sibling->state == PERF_EVENT_STATE_ACTIVE) - list_add_tail(&sibling->active_list, get_event_list(sibling)); - } + perf_promote_sibling_to_leader(sibling, ctx, event->group_caps); WARN_ON_ONCE(sibling->ctx != event->ctx); } + event->nr_siblings = 0; out: for_each_sibling_event(tmp, leader) @@ -2592,12 +2603,7 @@ __perf_remove_from_context(struct perf_event *event, if (flags & DETACH_DEAD) state = PERF_EVENT_STATE_DEAD; - event_sched_out(event, ctx); - - if (event->state > PERF_EVENT_STATE_OFF) - perf_cgroup_event_disable(event, ctx); - - perf_event_set_state(event, min(event->state, state)); + __event_disable(event, ctx, state); if (flags & DETACH_GROUP) perf_group_detach(event); @@ -2666,8 +2672,9 @@ static void __event_disable(struct perf_event *event, enum perf_event_state state) { event_sched_out(event, ctx); - perf_cgroup_event_disable(event, ctx); - perf_event_set_state(event, state); + if (event->state > PERF_EVENT_STATE_OFF) + perf_cgroup_event_disable(event, ctx); + perf_event_set_state(event, min(event->state, state)); } /*