[PATCH 2/2] taskstats: restrict access to user

[Vasiliy Kulikov <segoon@openwall.com>]

taskstats information may be used for gathering private information.
E.g. for openssh and vsftpd daemons read_characters/write_characters may
be used to learn the precise password length.  Restrict it to processes
being able to ptrace the target process.

For TASKSTATS_CMD_ATTR_REGISTER_CPUMASK the fix is euid check instead of
a ptrace check as the handler is processed in the context of the target
process, not the listener process'.  When ptrace_task_may_access_current()
is introduced, it should be used instead of euid check.  Currently there
is a small race when a process temporarily changes its euid (e.g. to
access user's files), until the process sets euid back user's processes
may gather privileged process' statistics.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
---
 kernel/taskstats.c |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)

diff --git a/kernel/taskstats.c b/kernel/taskstats.c
index 9ffea36..d92c95a 100644
--- a/kernel/taskstats.c
+++ b/kernel/taskstats.c
@@ -27,6 +27,7 @@
 #include <linux/cgroup.h>
 #include <linux/fs.h>
 #include <linux/file.h>
+#include <linux/ptrace.h>
 #include <net/genetlink.h>
 #include <asm/atomic.h>
 
@@ -132,6 +133,8 @@ static void send_cpu_listeners(struct sk_buff *skb,
 	struct sk_buff *skb_next, *skb_cur = skb;
 	void *reply = genlmsg_data(genlhdr);
 	int rc, delcount = 0;
+	const struct cred *cred = current_cred();
+	struct task_struct *task;
 
 	rc = genlmsg_end(skb, reply);
 	if (rc < 0) {
@@ -142,6 +145,15 @@ static void send_cpu_listeners(struct sk_buff *skb,
 	rc = 0;
 	down_read(&listeners->sem);
 	list_for_each_entry(s, &listeners->list, list) {
+
+		rcu_read_lock();
+		task = find_task_by_vpid(s->pid);
+		if (!task || __task_cred(task)->euid != cred->euid) {
+			rcu_read_unlock();
+			continue;
+		}
+		rcu_read_unlock();
+
 		skb_next = NULL;
 		if (!list_is_last(&s->list, &listeners->list)) {
 			skb_next = skb_clone(skb_cur, GFP_KERNEL);
@@ -199,14 +211,19 @@ static void fill_stats(struct task_struct *tsk, struct taskstats *stats)
 static int fill_stats_for_pid(pid_t pid, struct taskstats *stats)
 {
 	struct task_struct *tsk;
+	int rc = -ESRCH;
 
 	rcu_read_lock();
 	tsk = find_task_by_vpid(pid);
+	if (tsk && !ptrace_may_access(tsk, PTRACE_MODE_READ)) {
+		tsk = NULL;
+		rc = -EACCES;
+	}
 	if (tsk)
 		get_task_struct(tsk);
 	rcu_read_unlock();
 	if (!tsk)
-		return -ESRCH;
+		return rc;
 	fill_stats(tsk, stats);
 	put_task_struct(tsk);
 	return 0;
@@ -224,6 +241,10 @@ static int fill_stats_for_tgid(pid_t tgid, struct taskstats *stats)
 	 */
 	rcu_read_lock();
 	first = find_task_by_vpid(tgid);
+	if (first && !ptrace_may_access(first, PTRACE_MODE_READ)) {
+		rc = -EACCES;
+		goto out;
+	}
 
 	if (!first || !lock_task_sighand(first, &flags))
 		goto out;
-- 
1.7.0.4