[PATCH 4.19 024/114] proc: prevent changes to overridden credentials

[Greg Kroah-Hartman]

From: Paul Moore <paul@xxxxxxxxxxxxxx>

commit 35a196bef449b5824033865b963ed9a43fb8c730 upstream.

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@xxxxxxxxxxxxxxx>
Reported-by: "chengjian (D)" <cj.chengjian@xxxxxxxxxx>
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
Acked-by: John Johansen <john.johansen@xxxxxxxxxxxxx>
Acked-by: James Morris <james.morris@xxxxxxxxxxxxx>
Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/proc/base.c |    5 +++++
 1 file changed, 5 insertions(+)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -2542,6 +2542,11 @@ static ssize_t proc_pid_attr_write(struc
 		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)