[PATCH RFC v3 3/4] ptrace: add ptracer_access_allowed()

["Christian Brauner (Amutable)" <brauner@kernel.org>]

Add a helper that encapsulates all of the logic for checking ptrace
access and remove open-coded versions in follow-up patches.

Reviewed-by: Jann Horn <jannh@google.com>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
---
 include/linux/ptrace.h |  1 +
 kernel/ptrace.c        | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 90507d4afcd6..ef314f7a9ecc 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -17,6 +17,7 @@ struct syscall_info {
 	struct seccomp_data	data;
 };
 
+bool ptracer_access_allowed(struct task_struct *tsk);
 extern int ptrace_access_vm(struct task_struct *tsk, unsigned long addr,
 			    void *buf, int len, unsigned int gup_flags);
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 07398c9c8fe3..0e1f80f73a7f 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -13,6 +13,7 @@
 #include <linux/sched.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/coredump.h>
+#include <linux/sched/exec_state.h>
 #include <linux/sched/task.h>
 #include <linux/errno.h>
 #include <linux/mm.h>
@@ -36,6 +37,32 @@
 
 #include <asm/syscall.h>	/* for syscall_get_* */
 
+/**
+ * ptracer_access_allowed - may current peek/poke @tsk's address space?
+ * @tsk: tracee
+ *
+ * Per-access check used by ptrace_access_vm() and architecture-specific
+ * tag/register accessors.  Returns true iff current is the registered
+ * ptracer of @tsk and either @tsk is owner-dumpable or current holds
+ * CAP_SYS_PTRACE in @tsk's exec namespace.  Lighter than
+ * __ptrace_may_access(): it re-validates only dumpability and
+ * capability on every access, without re-running LSM hooks or
+ * cred_cap_issubset() checks performed at attach time.
+ */
+bool ptracer_access_allowed(struct task_struct *tsk)
+{
+	const struct task_exec_state *es;
+
+	if (!tsk->ptrace)
+		return false;
+	if (current != tsk->parent)
+		return false;
+	guard(rcu)();
+	es = task_exec_state_rcu(tsk);
+	return READ_ONCE(es->dumpable) == TASK_DUMPABLE_OWNER ||
+	       ptracer_capable(tsk, es->user_ns);
+}
+
 /*
  * Access another process' address space via ptrace.
  * Source/target buffer must be kernel space,

-- 
2.47.3