From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id ADF4E3B2FD4; Mon, 23 Mar 2026 14:10:04 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774275004; cv=none; b=dO2gYhjgFZWdSL861NV28G8pNWVUbr8IkJd3kap7pN2t49lc1RlOD6IIuIgAb4Ni80H786UGVxk17TbnHBTpRdSAEysWiQSetjUy9r9JP2OEpHIoXmjMejAO32h9CdOnCG3ixYLj6ZH/avuIOjFrsFwyrtT52ufm0gfYyokNbyw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1774275004; c=relaxed/simple; bh=7CGcG6Ba1/Hue0o67wba5zg7Okg0eQm70Gj10wRxfpI=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=EdoGXVBQKhmD7wLtLUvik20rHlGdSExzvByoY3Ac2MeGm4a/JjyA7cowbHRrHXQVerqXNGSESk4vxJSR+pbFjSDLKkx7pp5NE7OeNf5YdTyUfYyQXNdszj9qUn5Zy2AfhPKrNLZEpLCGuTGPMkKZd9q0CAH6nFrCoLzEuBOIxEs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=dEnGKHMQ; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="dEnGKHMQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 37C22C2BCB1; Mon, 23 Mar 2026 14:10:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1774275004; bh=7CGcG6Ba1/Hue0o67wba5zg7Okg0eQm70Gj10wRxfpI=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=dEnGKHMQi/kBTUB8YoUEzhXdOvZ7I9/ASCQvolbalugrglDDrg/G3IJdCkfOiUNp1 O+tRKiguPkCk2p+zEFsdanKKqQ++8sb+iREmVmWKkSjV+NfDqVc0No6LlxSRTGkgII z9qbgXZD4Dz2j8Wvik/laVuz00gUC/qikMy1VnUA= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Breno Leitao , "Peter Zijlstra (Intel)" Subject: [PATCH 6.18 190/212] perf/x86: Move event pointer setup earlier in x86_pmu_enable() Date: Mon, 23 Mar 2026 14:46:51 +0100 Message-ID: <20260323134509.740691924@linuxfoundation.org> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260323134503.770111826@linuxfoundation.org> References: <20260323134503.770111826@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore Precedence: bulk X-Mailing-List: stable@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.18-stable review patch. If anyone has any objections, please let me know. ------------------ From: Breno Leitao commit 8d5fae6011260de209aaf231120e8146b14bc8e0 upstream. A production AMD EPYC system crashed with a NULL pointer dereference in the PMU NMI handler: BUG: kernel NULL pointer dereference, address: 0000000000000198 RIP: x86_perf_event_update+0xc/0xa0 Call Trace: amd_pmu_v2_handle_irq+0x1a6/0x390 perf_event_nmi_handler+0x24/0x40 The faulting instruction is `cmpq $0x0, 0x198(%rdi)` with RDI=0, corresponding to the `if (unlikely(!hwc->event_base))` check in x86_perf_event_update() where hwc = &event->hw and event is NULL. drgn inspection of the vmcore on CPU 106 showed a mismatch between cpuc->active_mask and cpuc->events[]: active_mask: 0x1e (bits 1, 2, 3, 4) events[1]: 0xff1100136cbd4f38 (valid) events[2]: 0x0 (NULL, but active_mask bit 2 set) events[3]: 0xff1100076fd2cf38 (valid) events[4]: 0xff1100079e990a90 (valid) The event that should occupy events[2] was found in event_list[2] with hw.idx=2 and hw.state=0x0, confirming x86_pmu_start() had run (which clears hw.state and sets active_mask) but events[2] was never populated. Another event (event_list[0]) had hw.state=0x7 (STOPPED|UPTODATE|ARCH), showing it was stopped when the PMU rescheduled events, confirming the throttle-then-reschedule sequence occurred. The root cause is commit 7e772a93eb61 ("perf/x86: Fix NULL event access and potential PEBS record loss") which moved the cpuc->events[idx] assignment out of x86_pmu_start() and into step 2 of x86_pmu_enable(), after the PERF_HES_ARCH check. This broke any path that calls pmu->start() without going through x86_pmu_enable() -- specifically the unthrottle path: perf_adjust_freq_unthr_events() -> perf_event_unthrottle_group() -> perf_event_unthrottle() -> event->pmu->start(event, 0) -> x86_pmu_start() // sets active_mask but not events[] The race sequence is: 1. A group of perf events overflows, triggering group throttle via perf_event_throttle_group(). All events are stopped: active_mask bits cleared, events[] preserved (x86_pmu_stop no longer clears events[] after commit 7e772a93eb61). 2. While still throttled (PERF_HES_STOPPED), x86_pmu_enable() runs due to other scheduling activity. Stopped events that need to move counters get PERF_HES_ARCH set and events[old_idx] cleared. In step 2 of x86_pmu_enable(), PERF_HES_ARCH causes these events to be skipped -- events[new_idx] is never set. 3. The timer tick unthrottles the group via pmu->start(). Since commit 7e772a93eb61 removed the events[] assignment from x86_pmu_start(), active_mask[new_idx] is set but events[new_idx] remains NULL. 4. A PMC overflow NMI fires. The handler iterates active counters, finds active_mask[2] set, reads events[2] which is NULL, and crashes dereferencing it. Move the cpuc->events[hwc->idx] assignment in x86_pmu_enable() to before the PERF_HES_ARCH check, so that events[] is populated even for events that are not immediately started. This ensures the unthrottle path via pmu->start() always finds a valid event pointer. Fixes: 7e772a93eb61 ("perf/x86: Fix NULL event access and potential PEBS record loss") Signed-off-by: Breno Leitao Signed-off-by: Peter Zijlstra (Intel) Cc: stable@vger.kernel.org Link: https://patch.msgid.link/20260310-perf-v2-1-4a3156fce43c@debian.org Signed-off-by: Greg Kroah-Hartman --- arch/x86/events/core.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -1359,6 +1359,8 @@ static void x86_pmu_enable(struct pmu *p else if (i < n_running) continue; + cpuc->events[hwc->idx] = event; + if (hwc->state & PERF_HES_ARCH) continue; @@ -1366,7 +1368,6 @@ static void x86_pmu_enable(struct pmu *p * if cpuc->enabled = 0, then no wrmsr as * per x86_pmu_enable_event() */ - cpuc->events[hwc->idx] = event; x86_pmu_start(event, PERF_EF_RELOAD); } cpuc->n_added = 0;