proc: hold cred_guard_mutex in check_mem_permission()

Avoid a potential race when task exec's and we get a new ->mm but check against
the old credentials in ptrace_may_access().

Holding of the mutex is implemented by factoring out the body of the code into a
helper function __check_mem_permission().  Performing this factorization now
simplifies upcoming changes and minimizes churn in the diff's.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 22 insertions, 4 deletions

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 9af49a3984f1..013f116b3223 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -191,10 +191,7 @@ static int proc_root_link(struct inode *inode, struct path *path)
         return result;
 }
 
-/*
- * Return zero if current may access user memory in @task, -error if not.
- */
-static int check_mem_permission(struct task_struct *task)
+static int __check_mem_permission(struct task_struct *task)
 {
         /*
          * A task can always look at itself, in case it chooses
@@ -222,6 +219,27 @@ static int check_mem_permission(struct task_struct *task)
         return -EPERM;
 }
 
+/*
+ * Return zero if current may access user memory in @task, -error if not.
+ */
+static int check_mem_permission(struct task_struct *task)
+{
+        int err;
+
+        /*
+         * Avoid racing if task exec's as we might get a new mm but validate
+         * against old credentials.
+         */
+        err = mutex_lock_killable(&task->signal->cred_guard_mutex);
+        if (err)
+                return err;
+
+        err = __check_mem_permission(task);
+        mutex_unlock(&task->signal->cred_guard_mutex);
+
+        return err;
+}
+
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
         struct mm_struct *mm;