linux-kernel - Re: [PATCH] kmsg: Honor dmesg

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Message-Id: <20130322145448.f44f9d10a36620c1c11535b7@linux-foundation.org>
Date:	Fri, 22 Mar 2013 14:54:48 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	Josh Boyer <jwboyer@...hat.com>, Eric Paris <eparis@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christian Kujau <lists@...dbynature.de>,
	"# 3.4.x" <stable@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg

poke.  Nothing got applied.  I'll drop
kmsg-honor-dmesg_restrict-sysctl-on-dev-kmsg.patch, see if that has any
effect ;) 

From: Josh Boyer <jwboyer@...hat.com>
Subject: kmsg: honor dmesg_restrict sysctl on /dev/kmsg

Originally, the addition of dmesg_restrict covered both the syslog
method of accessing dmesg, as well as /dev/kmsg itself.  This was done
indirectly by security_syslog calling cap_syslog before doing any LSM
checks.

However, commit 12b3052c3ee ("capabilities/syslog: open code cap_syslog
logic to fix build failure") moved the code around and pushed the checks
into the caller itself.  That seems to have inadvertently dropped the
checks for dmesg_restrict on /dev/kmsg.  Most people haven't noticed
because util-linux dmesg(1) defaults to using the syslog method for access
in older versions.  With util-linux 2.22 and a kernel newer than 3.5,
dmesg(1) defaults to reading directly from /dev/kmsg.

Fix this by making an explicit check in the devkmsg_open function.

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192

Signed-off-by: Josh Boyer <jwboyer@...hat.com>
Reported-by: Christian Kujau <lists@...dbynature.de>
Cc: Eric Paris <eparis@...hat.com>
Cc: James Morris <jmorris@...ei.org>
Cc: <stable@...r.kernel.org>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 kernel/printk.c |    3 +++
 1 file changed, 3 insertions(+)

diff -puN kernel/printk.c~kmsg-honor-dmesg_restrict-sysctl-on-dev-kmsg kernel/printk.c
--- a/kernel/printk.c~kmsg-honor-dmesg_restrict-sysctl-on-dev-kmsg
+++ a/kernel/printk.c
@@ -620,6 +620,9 @@ static int devkmsg_open(struct inode *in
 	struct devkmsg_user *user;
 	int err;

+	if (dmesg_restrict && !capable(CAP_SYSLOG))
+		return -EACCES;
+
 	/* write-only does not need any file context */
 	if ((file->f_flags & O_ACCMODE) == O_WRONLY)
 		return 0;
_

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