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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Janis Danisevskis <jdanis@google.com>,
        Kees Cook <keescook@chromium.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Cyrill Gorcunov <gorcunov@openvz.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Colin Ian King <colin.king@canonical.com>,
        David Rientjes <rientjes@google.com>,
        Minfei Huang <mnfhuang@gmail.com>,
        John Stultz <john.stultz@linaro.org>,
        Calvin Owens <calvinowens@fb.com>, Jann Horn <jann@thejh.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: [PATCH 4.4 55/92] procfs: fix pthread cross-thread naming if !PR_DUMPABLE
Date:   Thu, 24 May 2018 11:38:32 +0200
Message-Id: <20180524093204.603778122@linuxfoundation.org>
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4.4-stable review patch.  If anyone has any objections, please let me know.

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

From: Janis Danisevskis <jdanis@google.com>

commit 1b3044e39a89cb1d4d5313da477e8dfea2b5232d upstream.

The PR_DUMPABLE flag causes the pid related paths of the proc file
system to be owned by ROOT.

The implementation of pthread_set/getname_np however needs access to
/proc/<pid>/task/<tid>/comm.  If PR_DUMPABLE is false this
implementation is locked out.

This patch installs a special permission function for the file "comm"
that grants read and write access to all threads of the same group
regardless of the ownership of the inode.  For all other threads the
function falls back to the generic inode permission check.

[akpm@linux-foundation.org: fix spello in comment]
Signed-off-by: Janis Danisevskis <jdanis@google.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Minfei Huang <mnfhuang@gmail.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Calvin Owens <calvinowens@fb.com>
Cc: Jann Horn <jann@thejh.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/proc/base.c |   42 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3083,6 +3083,44 @@ int proc_pid_readdir(struct file *file,
 }
 
 /*
+ * proc_tid_comm_permission is a special permission function exclusively
+ * used for the node /proc/<pid>/task/<tid>/comm.
+ * It bypasses generic permission checks in the case where a task of the same
+ * task group attempts to access the node.
+ * The rationale behind this is that glibc and bionic access this node for
+ * cross thread naming (pthread_set/getname_np(!self)). However, if
+ * PR_SET_DUMPABLE gets set to 0 this node among others becomes uid=0 gid=0,
+ * which locks out the cross thread naming implementation.
+ * This function makes sure that the node is always accessible for members of
+ * same thread group.
+ */
+static int proc_tid_comm_permission(struct inode *inode, int mask)
+{
+	bool is_same_tgroup;
+	struct task_struct *task;
+
+	task = get_proc_task(inode);
+	if (!task)
+		return -ESRCH;
+	is_same_tgroup = same_thread_group(current, task);
+	put_task_struct(task);
+
+	if (likely(is_same_tgroup && !(mask & MAY_EXEC))) {
+		/* This file (/proc/<pid>/task/<tid>/comm) can always be
+		 * read or written by the members of the corresponding
+		 * thread group.
+		 */
+		return 0;
+	}
+
+	return generic_permission(inode, mask);
+}
+
+static const struct inode_operations proc_tid_comm_inode_operations = {
+		.permission = proc_tid_comm_permission,
+};
+
+/*
  * Tasks
  */
 static const struct pid_entry tid_base_stuff[] = {
@@ -3100,7 +3138,9 @@ static const struct pid_entry tid_base_s
 #ifdef CONFIG_SCHED_DEBUG
 	REG("sched",     S_IRUGO|S_IWUSR, proc_pid_sched_operations),
 #endif
-	REG("comm",      S_IRUGO|S_IWUSR, proc_pid_set_comm_operations),
+	NOD("comm",      S_IFREG|S_IRUGO|S_IWUSR,
+			 &proc_tid_comm_inode_operations,
+			 &proc_pid_set_comm_operations, {}),
 #ifdef CONFIG_HAVE_ARCH_TRACEHOOK
 	ONE("syscall",   S_IRUSR, proc_pid_syscall),
 #endif