[PATCH] [RFC] cgroup: reject cgroup names with non-printing characters

[PATCH] [RFC] cgroup: reject cgroup names with non-printing characters

[Alban Crequy]

/proc/<pid>/cgroup contains one cgroup path on each line. If cgroup names are
allowed to contain "\n", applications cannot parse /proc/<pid>/cgroup safely.

I use < 0x20 as seen in vfat_bad_char; is it safe to use isprint()?

Signed-off-by: Alban Crequy <alban.crequy@collabora.co.uk>
---
 kernel/cgroup.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 70776ae..e2df5e7 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4378,6 +4378,20 @@ err_free_css:
 	return err;
 }
 
+/* Inspired by vfat_bad_char: do not accept non-printing characters. In
+ * particular, reject '\n' to prevent making /proc/<pid>/cgroup unparsable.
+ */
+static inline int cgroup_is_used_badchars(const char *s)
+{
+	int i;
+
+	for (i = 0; s[i] != '\0'; i++)
+		if (s[i] < 0x20)
+			return -EINVAL;
+
+	return 0;
+}
+
 static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 			umode_t mode)
 {
@@ -4387,6 +4401,11 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
 	struct kernfs_node *kn;
 	int ssid, ret;
 
+	/* do not create cgroups with bad names */
+	ret = cgroup_is_used_badchars(name);
+	if (ret)
+		return ret;
+
 	parent = cgroup_kn_lock_live(parent_kn);
 	if (!parent)
 		return -ENODEV;
-- 
1.8.5.3

Re: [PATCH] [RFC] cgroup: reject cgroup names with non-printing characters

[Tejun Heo]

Hello, Alban.

Sorry about the delay.

On Mon, Jul 21, 2014 at 04:11:56PM +0100, Alban Crequy wrote:
> @@ -4387,6 +4401,11 @@ static int cgroup_mkdir(struct kernfs_node *parent_kn, const char *name,
>  	struct kernfs_node *kn;
>  	int ssid, ret;
>  
> +	/* do not create cgroups with bad names */
> +	ret = cgroup_is_used_badchars(name);
> +	if (ret)
> +		return ret;

Let's just reject '\n'.  That should give us a way to parse the path
correctly and I don't want to put more-than-necessary interpretation
on the path string.

Thanks.

-- 
tejun