proc: actually make proc_fd_permission() thread-friendly

The commit 96d0df79f264 ("proc: make proc_fd_permission() thread-friendly")
fixed the access to /proc/self/fd from sub-threads, but introduced another
problem: a sub-thread can't access /proc/<tid>/fd/ or /proc/thread-self/fd
if generic_permission() fails.

Change proc_fd_permission() to check same_thread_group(pid_task(), current).

Fixes: 96d0df79f264 ("proc: make proc_fd_permission() thread-friendly")
Reported-by: "Jin, Yihua" <yihua.jin@intel.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 6e5fcd0..3c2a915 100644 (file)
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -291,11 +291,19 @@ static struct dentry *proc_lookupfd(struct inode *dir, struct dentry *dentry,
  */
 int proc_fd_permission(struct inode *inode, int mask)
 {
-       int rv = generic_permission(inode, mask);
+       struct task_struct *p;
+       int rv;
+
+       rv = generic_permission(inode, mask);
        if (rv == 0)
-               return 0;
-       if (task_tgid(current) == proc_pid(inode))
+               return rv;
+
+       rcu_read_lock();
+       p = pid_task(proc_pid(inode), PIDTYPE_PID);
+       if (p && same_thread_group(p, current))
                rv = 0;
+       rcu_read_unlock();
+
        return rv;
 }