Messages in this thread Patch in this message | | | Date | Tue, 7 Nov 2006 20:16:55 -0800 | From | "Zack Weinberg" <> | Subject | RFC PATCH: apply security_syslog() only to the syslog() syscall, not to /proc/kmsg |
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Presently, the security checks for syslog(2) apply also to access to /proc/kmsg, because /proc/kmsg's file_operations functions just call do_syslog, and the call to security_syslog is in do_syslog, not sys_syslog. [The only callers of do_syslog are sys_syslog and kmsg_{read,poll,open,release}.] This has the effect, with the default security policy, that no matter what the file permissions on /proc/kmsg are, only a process with CAP_SYS_ADMIN can actually open or read it. [Yes, if you open /proc/kmsg as root and then drop privileges, subsequent reads on that fd fail.] In consequence, if one wishes to run klogd as an unprivileged user, one is forced to jump through awkward hoops - for example, Ubuntu's /etc/init.d/klogd interposes a root-privileged "dd" process and a named pipe between /proc/kmsg and the actual klogd.
I propose to move the security_syslog() check from do_syslog to sys_syslog, so that the syscall remains restricted to CAP_SYS_ADMIN in the default policy, but /proc/kmsg is governed by its file permissions. With the attached patch, I can run klogd as an unprivileged user, having changed the ownership of /proc/kmsg to that user before starting it, and it still works. Equally, I can leave the ownership alone but modify klogd to get messages from stdin, start it with stdin open on /proc/kmsg (again unprivileged) and it works.
I think this is safe in the default security policy - /proc/kmsg starts out owned by root and mode 400 - but I am not sure of the impact on SELinux or other alternate policy frameworks.
Patch versus 2.6.19-rc4; please consider for the next release.
zw
Signed-off-by: Zack Weinberg <zackw@panix.com>
--- kernel/printk.c.unmod 2006-11-06 15:00:44.000000000 -0800 +++ kernel/printk.c 2006-11-06 15:01:51.000000000 -0800 @@ -187,10 +187,6 @@ int do_syslog(int type, char __user *buf char c; int error = 0;
- error = security_syslog(type); - if (error) - return error; - switch (type) { case 0: /* Close log */ break; @@ -317,6 +313,10 @@ out:
asmlinkage long sys_syslog(int type, char __user *buf, int len) { + int error = security_syslog(type); + if (error) + return error; + return do_syslog(type, buf, len); } - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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