[RFC PATCH v3 4/4] ipc: Allow to modify ipc/mq sysctls if CAP_SYS_RESOURCE is present

[Alexey Gladkov <legion@kernel.org>]

The mq_overview(7) says that mq sysctls are available for modification
to a privileged process (CAP_SYS_RESOURCE). Right now, within userns, a
privileged process cannot modify these files. Once the mq and ipc
sysctls have been moved to the ipc namespace we can grant access to
these files.

mqueue sysctls
--------------

The mq sysctls are protected by an upper limit that cannot be exceeded
on the system:

For /proc/sys/fs/mqueue/msg_max the upper limit is HARD_MSGMAX.
For /proc/sys/fs/mqueue/msgsize_max the upper limit is HARD_MSGSIZEMAX.

Also RLIMIT_MSGQUEUE limits all queues used by the process. This limit
is also tied to userns.

ipc sysctls
-----------

The implementation has no specific limits for the per-process maximum
number of shared memory segments. Only SHM_LOCK and SHM_HUGETLB limited
by RLIMIT_MEMLOCK which is also tied to userns.

This patch is RPC only and should not be applied without a security
discussion.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
---
 ipc/ipc_sysctl.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/ipc/ipc_sysctl.c b/ipc/ipc_sysctl.c
index 9fc8e3e75be7..f1d1c83656f9 100644
--- a/ipc/ipc_sysctl.c
+++ b/ipc/ipc_sysctl.c
@@ -254,6 +254,21 @@ static struct ctl_table mq_sysctls[] = {
 	{}
 };
 
+static int set_permissions(struct ctl_table_header *head, struct ctl_table *table)
+{
+	struct ipc_namespace *ns = container_of(head->set, struct ipc_namespace, set);
+	int mode;
+
+	/* Allow users with CAP_SYS_RESOURCE unrestrained access */
+	if (ns_capable(ns->user_ns, CAP_SYS_RESOURCE))
+		mode = (table->mode & S_IRWXU) >> 6;
+	else
+	/* Allow all others at most read-only access */
+		mode = table->mode & S_IROTH;
+
+	return (mode << 6) | (mode << 3) | mode;
+}
+
 static struct ctl_table_set *set_lookup(struct ctl_table_root *root)
 {
 	return &current->nsproxy->ipc_ns->set;
@@ -266,6 +281,7 @@ static int set_is_seen(struct ctl_table_set *set)
 
 static struct ctl_table_root set_root = {
 	.lookup = set_lookup,
+	.permissions = set_permissions,
 };
 
 static bool register_ipc_sysctl_table(struct ipc_namespace *ns)
-- 
2.33.0