Allow unprivileged users to trace their own processes' syscalls using perf trace, similar to strace without the overhead of ptrace().
Currently, perf trace requires CAP_PERFMON or paranoid level ≤ 1 even though the kernel has existing infrastructure (TRACE_EVENT_FL_CAP_ANY) designed to mark syscall tracepoints as safe for unprivileged access. To fix this: 1. Loosen the condition in perf_event_open() which requires privileges for all events with exclude_kernel=0. This allows perf_event_open() to bypass the paranoid check for task-attached tracepoint events. Ensure that sample types which can expose kernel addresses to unprivileged users are blocked. Ensure the PERF_SECURITY_KERNEL LSM hook is preserved. 2. Add a check to perf_trace_event_perm() to block PERF_SAMPLE_IP on kernel tracepoints for unprivileged users to prevent KASLR bypass. We do this here rather than in kaddr_leak because perf_trace_event_perm() can distinguish between kernel tracepoints and uprobe tracepoints, where the IP is a safe user space address and is necessary for uprobe functionality. 3. Restrict pure counting events (no PERF_SAMPLE_RAW) to TRACE_EVENT_FL_CAP_ANY tracepoints preventing unprivileged users from counting internal kernel tracepoints while preserving current behavior for exclude_kernel=1 events. Example usage after this change: $ perf trace ls # works as unprivileged user $ perf trace # system-wide, still requires privileges $ perf trace -p 1234 # requires ptrace permission on pid 1234 Assisted-by: CLAUDE:claude-opus-4 Apogee Signed-off-by: Anubhav Shelat <[email protected]> --- kernel/events/core.c | 28 +++++++++++++++++++++++++--- kernel/trace/trace_event_perf.c | 28 +++++++++++++++++++++++++++- 2 files changed, 52 insertions(+), 4 deletions(-) diff --git a/kernel/events/core.c b/kernel/events/core.c index 954c36e28101..48bfff07ae02 100644 --- a/kernel/events/core.c +++ b/kernel/events/core.c @@ -13910,9 +13910,31 @@ SYSCALL_DEFINE5(perf_event_open, return err; if (!attr.exclude_kernel) { - err = perf_allow_kernel(); - if (err) - return err; + bool tp_bypass = false; + + /* Check unprivileged tracepoints */ + if (attr.type == PERF_TYPE_TRACEPOINT && pid != -1) { + /* + * Block sample types that expose kernel addresses to + * prevent KASLR bypass + */ + u64 kaddr_leak = PERF_SAMPLE_CALLCHAIN | + PERF_SAMPLE_BRANCH_STACK | + PERF_SAMPLE_ADDR | + PERF_SAMPLE_REGS_INTR; + + tp_bypass = !(attr.sample_type & kaddr_leak); + } + + if (!tp_bypass) { + err = perf_allow_kernel(); + if (err) + return err; + } else { + err = security_perf_event_open(PERF_SECURITY_KERNEL); + if (err) + return err; + } } if (attr.namespaces) { diff --git a/kernel/trace/trace_event_perf.c b/kernel/trace/trace_event_perf.c index 5b272856e5ab..a264154b460e 100644 --- a/kernel/trace/trace_event_perf.c +++ b/kernel/trace/trace_event_perf.c @@ -24,6 +24,16 @@ typedef typeof(unsigned long [PERF_MAX_TRACE_SIZE / sizeof(unsigned long)]) /* Count the events in use (per event id, not per instance) */ static int total_ref_count; +/* Check if perf tracepoint is restricted for unprivileged users */ +static bool perf_tp_is_restricted(struct perf_event *p_event) +{ + if (p_event->attr.exclude_kernel) + return false; + if (sysctl_perf_event_paranoid <= 1 || perfmon_capable()) + return false; + return true; +} + static int perf_trace_event_perm(struct trace_event_call *tp_event, struct perf_event *p_event) { @@ -72,9 +82,25 @@ static int perf_trace_event_perm(struct trace_event_call *tp_event, return -EINVAL; } + /* + * PERF_SAMPLE_IP on kernel tracepoints exposes a kernel text + * address, weakening KASLR. Block for unprivileged users unless + * the tracepoint is a uprobe (userspace IP, safe to expose). + */ + if ((p_event->attr.sample_type & PERF_SAMPLE_IP) && + !(tp_event->flags & TRACE_EVENT_FL_UPROBE) && + perf_tp_is_restricted(p_event)) + return -EACCES; + /* No tracing, just counting, so no obvious leak */ - if (!(p_event->attr.sample_type & PERF_SAMPLE_RAW)) + if (!(p_event->attr.sample_type & PERF_SAMPLE_RAW)) { + /* Prevent unprivileged users from counting kernel tracepoints */ + if (perf_tp_is_restricted(p_event) && + !(p_event->attach_state == PERF_ATTACH_TASK && + (tp_event->flags & TRACE_EVENT_FL_CAP_ANY))) + return -EACCES; return 0; + } /* Some events are ok to be traced by non-root users... */ if (p_event->attach_state == PERF_ATTACH_TASK) { -- 2.54.0
