[PATCH 3.19.y-ckt 094/164] proc: actually make proc_fd_permission() thread-friendly

[Kamal Mostafa]

3.19.8-ckt11 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg at redhat.com>

commit 54708d2858e79a2bdda10bf8a20c80eb96c20613 upstream.

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 at intel.com>
Signed-off-by: Oleg Nesterov <oleg at redhat.com>
Cc: "Eric W. Biederman" <ebiederm at xmission.com>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 fs/proc/fd.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/proc/fd.c b/fs/proc/fd.c
index 8e5ad83..4b724b3 100644
--- a/fs/proc/fd.c
+++ b/fs/proc/fd.c
@@ -284,11 +284,19 @@ 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;
 }
 
-- 
1.9.1