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[15.235.184.193]) by smtp.gmail.com with ESMTPSA id 5a478bee46e88-36fa9fd0000sm4681361eec.21.2026.11.06.02.15.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 06 Nov 2026 02:15:44 -0800 (PST) From: Phat Dinh To: peterz@infradead.org, mingo@redhat.com, acme@kernel.org, namhyung@kernel.org Cc: security@kernel.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] perf/core: Do not resurrect cpuctx->cgrp after last cgroup event exit Date: Fri, 6 Nov 2026 17:16:42 +0700 Message-ID: <20261106101642.494674-1-phatdt1116@gmail.com> X-Mailer: git-send-email 2.56.0 X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit perf_cgroup_switch() relies on: /* * cpuctx->cgrp is set when the first cgroup event enabled, * and is cleared when the last cgroup event disabled. */ It computes the next task's cgroup, schedules out the CPU context and then assigns cpuctx->cgrp unconditionally: ctx_sched_out(&cpuctx->ctx, NULL, EVENT_ALL|EVENT_CGROUP); /* * must not be done before ctxswout due * to update_cgrp_time_from_cpuctx() in * ctx_sched_out() */ cpuctx->cgrp = cgrp; However, ctx_sched_out() can disable events itself. If a cgroup event has a pending disable from perf_event_disable_inatomic() (e.g. its PERF_EVENT_IOC_REFRESH limit ran out) that the irq_work has not handled yet, event_sched_out() consumes it inline: if (event->pending_disable) { event->pending_disable = 0; perf_cgroup_event_disable(event, ctx); state = PERF_EVENT_STATE_OFF; } If that was the last cgroup event, perf_cgroup_event_disable() clears cpuctx->cgrp: if (--ctx->nr_cgroups) return; cpuctx->cgrp = NULL; and perf_cgroup_switch() then sets it again. cpuctx->cgrp is left non-NULL with ctx.nr_cgroups == 0, so the NULL check in perf_cgroup_switch() no longer catches it and the next switch trips: WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0); cpuctx->cgrp does not hold a reference on the cgroup. Once that cgroup is removed, the next ctx_sched_in() dereferences the stale pointer: BUG: KASAN: slab-use-after-free in ctx_sched_in+0xb1f/0xfa0 Read of size 8 at addr ffff8881008972d0 by task swapper/3/0 Tainted: [W]=WARN Call Trace: ctx_sched_in+0xb1f/0xfa0 perf_event_sched_in+0x1a/0x70 ctx_resched+0x32b/0xac0 __perf_install_in_context+0x475/0xab0 remote_function+0x105/0x200 __flush_smp_call_function_queue+0x179/0x8d0 __sysvec_call_function_single+0x1b/0x1c0 sysvec_call_function_single+0x66/0x80 asm_sysvec_call_function_single+0x1a/0x20 Only assign cpuctx->cgrp when cgroup events remain after the context has been scheduled out, so that it stays NULL as the invariant requires. Fixes: e19cd0b6fa59 ("perf/core: Always set cpuctx cgrp when enable cgroup event") Cc: stable@vger.kernel.org Signed-off-by: Phat Dinh --- kernel/events/core.c | 7 ++++++- 1 file changed, 6 insertions(+), 1 deletion(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index f36a3bbaa33c..54b87584c752 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -1072,8 +1072,13 @@ static void perf_cgroup_switch(struct task_struct *task) * must not be done before ctxswout due * to update_cgrp_time_from_cpuctx() in * ctx_sched_out() + * + * ctx_sched_out() can consume a pending disable and drop the last cgroup + * event, which clear the cpuctx->cgrp. Cgroup tracking follow active cgroup + * event, so do not resurrect it */ - cpuctx->cgrp = cgrp; + if (cpuctx->ctx.nr_cgroups) + cpuctx->cgrp = cgrp; /* * set cgrp before ctxsw in to allow * perf_cgroup_set_timestamp() in ctx_sched_in() base-commit: d095321349222961813333277881b3c74f10b17b -- 2.56.0