procfs: block chmod on /proc/thread-self/comm

Due to an oversight in commit 1b3044e ("procfs: fix pthread
cross-thread naming if !PR_DUMPABLE") in switching from REG to NOD,
chmod operations on /proc/thread-self/comm were no longer blocked as
they are on almost all other procfs files.

A very similar situation with /proc/self/environ was used to as a root
exploit a long time ago, but procfs has SB_I_NOEXEC so this is simply a
correctness issue.

Ref: https://lwn.net/Articles/191954/
Ref: 6d76fa5 ("Don't allow chmod() on the /proc/<pid>/ files")
Fixes: 1b3044e ("procfs: fix pthread cross-thread naming if !PR_DUMPABLE")
Cc: [email protected] # v4.7+
Signed-off-by: Aleksa Sarai <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: cyphar

fs/proc/base.c

@@ -3583,7 +3583,8 @@ static int proc_tid_comm_permission(struct mnt_idmap *idmap,
 }
 
 static const struct inode_operations proc_tid_comm_inode_operations = {
-		.permission = proc_tid_comm_permission,
+		.setattr	= proc_setattr,
+		.permission	= proc_tid_comm_permission,
 };
 
 /*