CRED: Use RCU to access another task's creds and to release a task's own creds

Use RCU to access another task's creds and to release a task's own creds.
This means that it will be possible for the credentials of a task to be
replaced without another task (a) requiring a full lock to read them, and (b)
seeing deallocated memory.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: James Morris <jmorris@namei.org>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 4 insertions, 3 deletions

diff --git a/kernel/futex_compat.c b/kernel/futex_compat.c
index 2c3fd5ed34f5..d607a5b9ee29 100644
--- a/kernel/futex_compat.c
+++ b/kernel/futex_compat.c
@@ -135,7 +135,7 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr,
 {
         struct compat_robust_list_head __user *head;
         unsigned long ret;
-        uid_t euid = current_euid();
+        const struct cred *cred = current_cred(), *pcred;
 
         if (!futex_cmpxchg_enabled)
                 return -ENOSYS;
@@ -151,8 +151,9 @@ compat_sys_get_robust_list(int pid, compat_uptr_t __user *head_ptr,
                 if (!p)
                         goto err_unlock;
                 ret = -EPERM;
-                if (euid != p->cred->euid &&
-                    euid != p->cred->uid &&
+                pcred = __task_cred(p);
+                if (cred->euid != pcred->euid &&
+                    cred->euid != pcred->uid &&
                     !capable(CAP_SYS_PTRACE))
                         goto err_unlock;
                 head = p->compat_robust_list;