procfs: prevent unprivileged processes accessing fdinfo dir

The file permissions on the fdinfo dir from were changed from
S_IRUSR|S_IXUSR to S_IRUGO|S_IXUGO, and a PTRACE_MODE_READ check was added
for opening the fdinfo files [1].  However, the ptrace permission check
was not added to the directory, allowing anyone to get the open FD numbers
by reading the fdinfo directory.

Add the missing ptrace permission check for opening the fdinfo directory.

[1] https://lkml.kernel.org/r/[email protected]

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 7bc3fa0 ("procfs: allow reading fdinfo with PTRACE_MODE_READ")
Signed-off-by: Kalesh Singh <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Eric W. Biederman <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Hridya Valsaraju <[email protected]>
Cc: Jann Horn <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Kalesh Singh

fs/proc/fd.c

@@ -72,7 +72,7 @@ static int seq_show(struct seq_file *m, void *v)
 	return 0;
 }
 
-static int seq_fdinfo_open(struct inode *inode, struct file *file)
+static int proc_fdinfo_access_allowed(struct inode *inode)
 {
 	bool allowed = false;
 	struct task_struct *task = get_proc_task(inode);
@@ -86,6 +86,16 @@ static int seq_fdinfo_open(struct inode *inode, struct file *file)
 	if (!allowed)
 		return -EACCES;
 
+	return 0;
+}
+
+static int seq_fdinfo_open(struct inode *inode, struct file *file)
+{
+	int ret = proc_fdinfo_access_allowed(inode);
+
+	if (ret)
+		return ret;
+
 	return single_open(file, seq_show, inode);
 }
 
@@ -348,12 +358,23 @@ static int proc_readfdinfo(struct file *file, struct dir_context *ctx)
 				  proc_fdinfo_instantiate);
 }
 
+static int proc_open_fdinfo(struct inode *inode, struct file *file)
+{
+	int ret = proc_fdinfo_access_allowed(inode);
+
+	if (ret)
+		return ret;
+
+	return 0;
+}
+
 const struct inode_operations proc_fdinfo_inode_operations = {
 	.lookup		= proc_lookupfdinfo,
 	.setattr	= proc_setattr,
 };
 
 const struct file_operations proc_fdinfo_operations = {
+	.open		= proc_open_fdinfo,
 	.read		= generic_read_dir,
 	.iterate_shared	= proc_readfdinfo,
 	.llseek		= generic_file_llseek,