proc: prevent changes to overridden credentials

Prevent userspace from changing the the /proc/PID/attr values if the
task's credentials are currently overriden.  This not only makes sense
conceptually, it also prevents some really bizarre error cases caused
when trying to commit credentials to a task with overridden
credentials.

Cc: <stable@vger.kernel.org>
Reported-by: "chengjian (D)" <cj.chengjian@huawei.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Acked-by: James Morris <james.morris@microsoft.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ddef482f1334..87ba007b86db 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2539,6 +2539,11 @@ static ssize_t proc_pid_attr_write(struct file * file, const char __user * buf,
                 rcu_read_unlock();
                 return -EACCES;
         }
+        /* Prevent changes to overridden credentials. */
+        if (current_cred() != current_real_cred()) {
+                rcu_read_unlock();
+                return -EBUSY;
+        }
         rcu_read_unlock();
 
         if (count > PAGE_SIZE)