Re: [PATCH] kmsg: Honor dmesg_restrict sysctl on /dev/kmsg

[Andrew Morton]

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@xxxxxxxxxx>
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@xxxxxxxxxx>
Reported-by: Christian Kujau <lists@xxxxxxxxxxxxxxx>
Cc: Eric Paris <eparis@xxxxxxxxxx>
Cc: James Morris <jmorris@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 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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