[PATCH RFC] perf: Allow restricted kernel breakpoints on user addresses

[Marco Elver]

Allow the creation of restricted breakpoint perf events that also fire
in the kernel (!exclude_kernel), if:

  1. No sample information is requested; samples may contain IPs,
     registers, or other information that may disclose kernel addresses.

  2. The breakpoint (viz. data watchpoint) is on a user address.

The rules constrain the allowable perf events such that no sensitive
kernel information can be disclosed.

Despite no explicit kernel information disclosure, the following
questions may need answers:

	1. Is obtaining information that the kernel accessed a
	   particular user's known memory location revealing new
	   information? Given the kernel's user space ABI, there should
	   be no "surprise accesses" to user space memory in the first
	   place.

	2. Does causing breakpoints on user memory accesses by the
	   kernel potentially impact timing in a sensitive way? Given
	   that hardware breakpoints trigger regardless of the state of
	   perf_event_attr::exclude_kernel, but are filtered in the perf
	   subsystem, this possibility already exists independent of the
	   proposed change.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
---

RFC:

We are looking to identify a set of constraints on perf events that
would allow them to safely be created by unprivileged users when
perf_event_paranoid > 1 && !perfmon_capable().

Our current (and only) event type of interest is PERF_TYPE_BREAKPOINT.

Any thoughts?

---
 include/linux/perf_event.h |  8 +-------
 kernel/events/core.c       | 38 ++++++++++++++++++++++++++++++++++++++
 2 files changed, 39 insertions(+), 7 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index af97dd427501..06c2ed46cbf9 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1348,13 +1348,7 @@ static inline int perf_is_paranoid(void)
 	return sysctl_perf_event_paranoid > -1;
 }
 
-static inline int perf_allow_kernel(struct perf_event_attr *attr)
-{
-	if (sysctl_perf_event_paranoid > 1 && !perfmon_capable())
-		return -EACCES;
-
-	return security_perf_event_open(attr, PERF_SECURITY_KERNEL);
-}
+extern int perf_allow_kernel(struct perf_event_attr *attr);
 
 static inline int perf_allow_cpu(struct perf_event_attr *attr)
 {
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 6eafb1b0ad4a..f37047cfcb2f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3266,6 +3266,12 @@ static int perf_event_modify_attr(struct perf_event *event,
 		return -EOPNOTSUPP;
 	}
 
+	if (!event->attr.exclude_kernel) {
+		err = perf_allow_kernel(attr);
+		if (err)
+			return err;
+	}
+
 	WARN_ON_ONCE(event->ctx->parent_ctx);
 
 	mutex_lock(&event->child_mutex);
@@ -12104,6 +12110,38 @@ perf_check_permission(struct perf_event_attr *attr, struct task_struct *task)
 	return is_capable || ptrace_may_access(task, ptrace_mode);
 }
 
+/*
+ * Check if unprivileged users are allowed to set up breakpoints on user
+ * addresses that also count when the kernel accesses them.
+ */
+static bool perf_allow_kernel_breakpoint(struct perf_event_attr *attr)
+{
+	if (attr->type != PERF_TYPE_BREAKPOINT)
+		return false;
+
+	/*
+	 * The sample may contain IPs, registers, or other information that may
+	 * disclose kernel addresses or timing information. Disallow any kind of
+	 * additional sample information.
+	 */
+	if (attr->sample_type)
+		return false;
+
+	/*
+	 * Only allow kernel breakpoints on user addresses.
+	 */
+	return access_ok((void __user *)(unsigned long)attr->bp_addr, attr->bp_len);
+}
+
+int perf_allow_kernel(struct perf_event_attr *attr)
+{
+	if (sysctl_perf_event_paranoid > 1 && !perfmon_capable() &&
+	    !perf_allow_kernel_breakpoint(attr))
+		return -EACCES;
+
+	return security_perf_event_open(attr, PERF_SECURITY_KERNEL);
+}
+
 /**
  * sys_perf_event_open - open a performance event, associate it to a task/cpu
  *
-- 
2.36.1.255.ge46751e96f-goog