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>

1 files changed, 11 insertions, 3 deletions

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 6e5fcd00733e..3c2a915c695a 100644
--- 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;
 }