CRED: Guard the setprocattr security hook against ptrace

Guard the setprocattr security hook against ptrace by taking the target task's
cred_guard_mutex around it.  The problem is that setprocattr() may otherwise
note the lack of a debugger, and then perform an action on that basis whilst
letting a debugger attach between the two points.  Holding cred_guard_mutex
across the test and the action prevents ptrace_attach() from doing that.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index fb45615943c2..23342e188a66 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2128,9 +2128,15 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
         if (copy_from_user(page, buf, count))
                 goto out_free;
 
+        /* Guard against adverse ptrace interaction */
+        length = mutex_lock_interruptible(&task->cred_guard_mutex);
+        if (length < 0)
+                goto out_free;
+
         length = security_setprocattr(task,
                                       (char*)file->f_path.dentry->d_name.name,
                                       (void*)page, count);
+        mutex_unlock(&task->cred_guard_mutex);
 out_free:
         free_page((unsigned long) page);
 out: