[PATCH 9/9] procfs: improve /proc/<pid>/stack protection

[Djalal Harouni]

Convert stack from an INF entry to a REG one. This way we can perform
and cache the permission checks during ->open(). We make sure that
/proc/<pid>/stack will continue to use sequence iterators.

The ptrace capability is only cached, it will be re-checked during
->read(). If the opener did not have enough privileges then fail.

Signed-off-by: Djalal Harouni <tixxdz@xxxxxxxxxx>
---
 fs/proc/base.c | 59 +++++++++++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 54 insertions(+), 5 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index d98ce15..6786878 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -387,14 +387,19 @@ static void unlock_trace(struct task_struct *task)
 
 #define MAX_STACK_TRACE_DEPTH	64
 
-static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
-			  struct pid *pid, struct task_struct *task)
+static int proc_show_pid_stack(struct seq_file *m, struct pid_namespace *ns,
+			       struct pid *pid, struct task_struct *task)
 {
 	struct stack_trace trace;
 	unsigned long *entries;
-	int err;
+	struct pid_seq_private *priv = m->private;
+	int permitted = priv->permitted;
+	int err = -EPERM;
 	int i;
 
+	if (!permitted)
+		return err;
+
 	entries = kmalloc(MAX_STACK_TRACE_DEPTH * sizeof(*entries), GFP_KERNEL);
 	if (!entries)
 		return -ENOMEM;
@@ -418,6 +423,50 @@ static int proc_pid_stack(struct seq_file *m, struct pid_namespace *ns,
 
 	return err;
 }
+
+static int proc_pid_stack(struct seq_file *m, void *v)
+{
+	return pid_entry_show(m, proc_show_pid_stack);
+}
+
+static int stack_open(struct inode *inode, struct file *filp)
+{
+	int ret = -ENOMEM;
+	struct pid_seq_private *priv;
+
+	priv = kzalloc(sizeof(*priv), GFP_KERNEL);
+	if (!priv)
+		return ret;
+
+	priv->inode = inode;
+	if (pid_entry_access(filp, PTRACE_MODE_ATTACH))
+		priv->permitted = PID_ENTRY_DENY;
+	else
+		priv->permitted = PID_ENTRY_ALLOW;
+
+	ret = single_open(filp, proc_pid_stack, priv);
+	if (ret)
+		kfree(priv);
+
+	return ret;
+}
+
+static int stack_release(struct inode *inode, struct file *filp)
+{
+	struct seq_file *seq = filp->private_data;
+
+	kfree(seq->private);
+	seq->private = NULL;
+
+	return single_release(inode, filp);
+}
+
+static const struct file_operations proc_pid_stack_operations = {
+	.open		= stack_open,
+	.read           = seq_read,
+	.llseek         = seq_lseek,
+	.release        = stack_release,
+};
 #endif
 
 #ifdef CONFIG_SCHEDSTATS
@@ -2746,7 +2795,7 @@ static const struct pid_entry tgid_base_stuff[] = {
 	REG("wchan",      S_IRUGO, proc_pid_wchan_operations),
 #endif
 #ifdef CONFIG_STACKTRACE
-	ONE("stack",      S_IRUSR, proc_pid_stack),
+	REG("stack",      S_IRUSR, proc_pid_stack_operations),
 #endif
 #ifdef CONFIG_SCHEDSTATS
 	INF("schedstat",  S_IRUGO, proc_pid_schedstat),
@@ -3082,7 +3131,7 @@ static const struct pid_entry tid_base_stuff[] = {
 	REG("wchan",     S_IRUGO, proc_pid_wchan_operations),
 #endif
 #ifdef CONFIG_STACKTRACE
-	ONE("stack",      S_IRUSR, proc_pid_stack),
+	REG("stack",      S_IRUSR, proc_pid_stack_operations),
 #endif
 #ifdef CONFIG_SCHEDSTATS
 	INF("schedstat", S_IRUGO, proc_pid_schedstat),
-- 
1.7.11.7

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