linux-kernel - [23/55] sysctl: restrict write access to dmesg

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Date:	Fri, 25 Mar 2011 16:47:16 -0700
From:	Greg KH <gregkh@...e.de>
To:	linux-kernel@...r.kernel.org, stable@...nel.org
Cc:	stable-review@...nel.org, torvalds@...ux-foundation.org,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	Richard Weinberger <richard@....at>,
	Dan Rosenberg <drosenberg@...curity.com>,
	"Serge E. Hallyn" <serge@...lyn.com>,
	Eric Paris <eparis@...hat.com>,
	Kees Cook <kees.cook@...onical.com>,
	James Morris <jmorris@...ei.org>,
	Eugene Teo <eugeneteo@...nel.org>
Subject: [23/55] sysctl: restrict write access to dmesg_restrict

2.6.37-stable review patch.  If anyone has any objections, please let us know.

------------------

From: Richard Weinberger <richard@....at>

commit bfdc0b497faa82a0ba2f9dddcf109231dd519fcc upstream.

When dmesg_restrict is set to 1 CAP_SYS_ADMIN is needed to read the kernel
ring buffer.  But a root user without CAP_SYS_ADMIN is able to reset
dmesg_restrict to 0.

This is an issue when e.g.  LXC (Linux Containers) are used and complete
user space is running without CAP_SYS_ADMIN.  A unprivileged and jailed
root user can bypass the dmesg_restrict protection.

With this patch writing to dmesg_restrict is only allowed when root has
CAP_SYS_ADMIN.

Signed-off-by: Richard Weinberger <richard@....at>
Acked-by: Dan Rosenberg <drosenberg@...curity.com>
Acked-by: Serge E. Hallyn <serge@...lyn.com>
Cc: Eric Paris <eparis@...hat.com>
Cc: Kees Cook <kees.cook@...onical.com>
Cc: James Morris <jmorris@...ei.org>
Cc: Eugene Teo <eugeneteo@...nel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

---
 kernel/sysctl.c |   18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -168,6 +168,11 @@ static int proc_taint(struct ctl_table *
 			       void __user *buffer, size_t *lenp, loff_t *ppos);
 #endif

+#ifdef CONFIG_PRINTK
+static int proc_dmesg_restrict(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp, loff_t *ppos);
+#endif
+
 #ifdef CONFIG_MAGIC_SYSRQ
 /* Note: sysrq code uses it's own private copy */
 static int __sysrq_enabled = SYSRQ_DEFAULT_ENABLE;
@@ -1469,7 +1474,7 @@ static struct ctl_table fs_table[] = {
 		.data		= &suid_dumpable,
 		.maxlen		= sizeof(int),
 		.mode		= 0644,
-		.proc_handler	= proc_dointvec_minmax,
+		.proc_handler	= proc_dmesg_restrict,
 		.extra1		= &zero,
 		.extra2		= &two,
 	},
@@ -2400,6 +2405,17 @@ static int proc_taint(struct ctl_table *
 	return err;
 }

+#ifdef CONFIG_PRINTK
+static int proc_dmesg_restrict(struct ctl_table *table, int write,
+				void __user *buffer, size_t *lenp, loff_t *ppos)
+{
+	if (write && !capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	return proc_dointvec_minmax(table, write, buffer, lenp, ppos);
+}
+#endif
+
 struct do_proc_dointvec_minmax_conv_param {
 	int *min;
 	int *max;

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