linux-kernel - [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be applied on arbitrary tasks

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Message-ID: <nycvar.YFH.7.76.1809041623230.15880@cbobk.fhfr.pm>
Date:   Tue, 4 Sep 2018 16:24:01 +0200 (CEST)
From:   Jiri Kosina <jikos@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be
 applied on arbitrary tasks

From: Jiri Kosina <jkosina@...e.cz>

Current ptrace_may_access() implementation assumes that the 'source' task is
always the caller (current).

Expose ___ptrace_may_access() that can be used to apply the check on arbitrary
tasks.

Originally-by: Tim Chen <tim.c.chen@...ux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@...e.cz>
---
 include/linux/ptrace.h | 12 ++++++++++++
 kernel/ptrace.c        | 13 ++++++++++---
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 4f36431c380b..09744d4113fb 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -87,6 +87,18 @@ extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
  */
 extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
 
+/**
+ * ___ptrace_may_access - variant of ptrace_may_access that can be used
+ * between two arbitrary tasks
+ * @curr: source task
+ * @task: target task
+ * @mode: selects type of access and caller credentials
+ *
+ * Returns true on success, false on denial.
+ */
+extern int ___ptrace_may_access(struct task_struct *curr, struct task_struct *task,
+				unsigned int mode);
+
 static inline int ptrace_reparented(struct task_struct *child)
 {
 	return !same_thread_group(child->real_parent, child->parent);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 21fec73d45d4..07ff6685ebed 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -268,9 +268,10 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
 }
 
 /* Returns 0 on success, -errno on denial. */
-static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
+int ___ptrace_may_access(struct task_struct *curr, struct task_struct *task,
+		unsigned int mode)
 {
-	const struct cred *cred = current_cred(), *tcred;
+	const struct cred *cred, *tcred;
 	struct mm_struct *mm;
 	kuid_t caller_uid;
 	kgid_t caller_gid;
@@ -290,9 +291,10 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	 */
 
 	/* Don't let security modules deny introspection */
-	if (same_thread_group(task, current))
+	if (same_thread_group(task, curr))
 		return 0;
 	rcu_read_lock();
+	cred =  __task_cred(curr);
 	if (mode & PTRACE_MODE_FSCREDS) {
 		caller_uid = cred->fsuid;
 		caller_gid = cred->fsgid;
@@ -331,6 +333,11 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	return security_ptrace_access_check(task, mode);
 }
 
+static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
+{
+	return ___ptrace_may_access(current, task, mode);
+}
+
 bool ptrace_may_access(struct task_struct *task, unsigned int mode)
 {
 	int err;

-- 
Jiri Kosina
SUSE Labs