proc: restrict access to /proc/PID/io

/proc/PID/io may be used for gathering private information.  E.g.  for
openssh and vsftpd daemons wchars/rchars may be used to learn the
precise password length.  Restrict it to processes being able to ptrace
the target process.

ptrace_may_access() is needed to prevent keeping open file descriptor of
"io" file, executing setuid binary and gathering io information of the
setuid'ed process.
Signed-off-by: NVasiliy Kulikov <segoon@openwall.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2708,6 +2708,9 @@ static int do_io_accounting(struct task_struct *task, char *buffer, int whole)
 	struct task_io_accounting acct = task->ioac;
 	unsigned long flags;
 
+	if (!ptrace_may_access(task, PTRACE_MODE_READ))
+		return -EACCES;
+
 	if (whole && lock_task_sighand(task, &flags)) {
 		struct task_struct *t = task;
 
@@ -2839,7 +2842,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 	REG("coredump_filter", S_IRUGO|S_IWUSR, proc_coredump_filter_operations),
 #endif
 #ifdef CONFIG_TASK_IO_ACCOUNTING
-	INF("io",	S_IRUGO, proc_tgid_io_accounting),
+	INF("io",	S_IRUSR, proc_tgid_io_accounting),
 #endif
 #ifdef CONFIG_HARDWALL
 	INF("hardwall",   S_IRUGO, proc_pid_hardwall),
@@ -3181,7 +3184,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	REG("make-it-fail", S_IRUGO|S_IWUSR, proc_fault_inject_operations),
 #endif
 #ifdef CONFIG_TASK_IO_ACCOUNTING
-	INF("io",	S_IRUGO, proc_tid_io_accounting),
+	INF("io",	S_IRUSR, proc_tid_io_accounting),
 #endif
 #ifdef CONFIG_HARDWALL
 	INF("hardwall",   S_IRUGO, proc_pid_hardwall),