[PATCH] [Xenial] procfs: fix pthread cross-thread naming if !PR_DUMPABLE

Breno Leitao leitao at debian.org
Thu May 11 21:10:19 UTC 2017

From: Janis Danisevskis <jdanis at google.com>

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1690225

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 at linux-foundation.org: fix spello in comment]
Signed-off-by: Janis Danisevskis <jdanis at google.com>
Acked-by: Kees Cook <keescook at chromium.org>
Cc: Al Viro <viro at zeniv.linux.org.uk>
Cc: Cyrill Gorcunov <gorcunov at openvz.org>
Cc: Alexey Dobriyan <adobriyan at gmail.com>
Cc: Colin Ian King <colin.king at canonical.com>
Cc: David Rientjes <rientjes at google.com>
Cc: Minfei Huang <mnfhuang at gmail.com>
Cc: John Stultz <john.stultz at linaro.org>
Cc: Calvin Owens <calvinowens at fb.com>
Cc: Jann Horn <jann at thejh.net>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(cherry picked from commit 1b3044e39a89cb1d4d5313da477e8dfea2b5232d)
Signed-off-by: Breno Leitao <breno.leitao at gmail.com>
 fs/proc/base.c | 42 +++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 41 insertions(+), 1 deletion(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index b4acb8837c79..cfc87edd7adf 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3081,6 +3081,44 @@ int proc_pid_readdir(struct file *file, struct dir_context *ctx)
+ * 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[] = {
@@ -3098,7 +3136,9 @@ static const struct pid_entry tid_base_stuff[] = {
 	REG("sched",     S_IRUGO|S_IWUSR, proc_pid_sched_operations),
-	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, {}),
 	ONE("syscall",   S_IRUSR, proc_pid_syscall),

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