ipc: check checkpoint_restore_ns_capable() to modify C/R proc files

This commit removes the requirement to be root to modify sem_next_id,
msg_next_id and shm_next_id and checks checkpoint_restore_ns_capable
instead.

Since those files are specific to the IPC namespace, there is no reason
they should require root privileges.  This is similar to ns_last_pid,
which also only checks checkpoint_restore_ns_capable.

[akpm@linux-foundation.org: ipc/ipc_sysctl.c needs capability.h for checkpoint_restore_ns_capable()]

Link: https://lkml.kernel.org/r/20210916163717.3179496-1-mclapinski@google.comSigned-off-by: Michal Clapinski <mclapinski@google.com>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Reviewed-by: Manfred Spraul <manfred@colorfullife.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/ipc_sysctl.c

@@ -10,6 +10,7 @@
 #include <linux/nsproxy.h>
 #include <linux/sysctl.h>
 #include <linux/uaccess.h>
+#include <linux/capability.h>
 #include <linux/ipc_namespace.h>
 #include <linux/msg.h>
 #include "util.h"
@@ -104,6 +105,19 @@ static int proc_ipc_sem_dointvec(struct ctl_table *table, int write,
 	return ret;
 }
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+static int proc_ipc_dointvec_minmax_checkpoint_restore(struct ctl_table *table,
+		int write, void *buffer, size_t *lenp, loff_t *ppos)
+{
+	struct user_namespace *user_ns = current->nsproxy->ipc_ns->user_ns;
+
+	if (write && !checkpoint_restore_ns_capable(user_ns))
+		return -EPERM;
+
+	return proc_ipc_dointvec_minmax(table, write, buffer, lenp, ppos);
+}
+#endif
+
 #else
 #define proc_ipc_doulongvec_minmax NULL
 #define proc_ipc_dointvec	   NULL
@@ -111,6 +125,9 @@ static int proc_ipc_sem_dointvec(struct ctl_table *table, int write,
 #define proc_ipc_dointvec_minmax_orphans   NULL
 #define proc_ipc_auto_msgmni	   NULL
 #define proc_ipc_sem_dointvec	   NULL
+#ifdef CONFIG_CHECKPOINT_RESTORE
+#define proc_ipc_dointvec_minmax_checkpoint_restore	NULL
+#endif	/* CONFIG_CHECKPOINT_RESTORE */
 #endif
 
 int ipc_mni = IPCMNI;
@@ -198,8 +215,8 @@ static struct ctl_table ipc_kern_table[] = {
 		.procname	= "sem_next_id",
 		.data		= &init_ipc_ns.ids[IPC_SEM_IDS].next_id,
 		.maxlen		= sizeof(init_ipc_ns.ids[IPC_SEM_IDS].next_id),
-		.mode		= 0644,
-		.proc_handler	= proc_ipc_dointvec_minmax,
+		.mode		= 0666,
+		.proc_handler	= proc_ipc_dointvec_minmax_checkpoint_restore,
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_INT_MAX,
 	},
@@ -207,8 +224,8 @@ static struct ctl_table ipc_kern_table[] = {
 		.procname	= "msg_next_id",
 		.data		= &init_ipc_ns.ids[IPC_MSG_IDS].next_id,
 		.maxlen		= sizeof(init_ipc_ns.ids[IPC_MSG_IDS].next_id),
-		.mode		= 0644,
-		.proc_handler	= proc_ipc_dointvec_minmax,
+		.mode		= 0666,
+		.proc_handler	= proc_ipc_dointvec_minmax_checkpoint_restore,
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_INT_MAX,
 	},
@@ -216,8 +233,8 @@ static struct ctl_table ipc_kern_table[] = {
 		.procname	= "shm_next_id",
 		.data		= &init_ipc_ns.ids[IPC_SHM_IDS].next_id,
 		.maxlen		= sizeof(init_ipc_ns.ids[IPC_SHM_IDS].next_id),
-		.mode		= 0644,
-		.proc_handler	= proc_ipc_dointvec_minmax,
+		.mode		= 0666,
+		.proc_handler	= proc_ipc_dointvec_minmax_checkpoint_restore,
 		.extra1		= SYSCTL_ZERO,
 		.extra2		= SYSCTL_INT_MAX,
 	},