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Message-Id: <1189688312.18713.17.camel@moss-spartans.epoch.ncsc.mil>
Date: Thu, 13 Sep 2007 08:58:32 -0400
From: Stephen Smalley <sds@...ho.nsa.gov>
To: Yuichi Nakamura <ynakam@...achisoft.jp>
Cc: selinux@...ho.nsa.gov, busybox@...gai.gr.jp,
James Morris <jmorris@...ei.org>,
Eric Paris <eparis@...isplace.org>, kaigai@...jp.nec.com,
linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC]selinux: Improving SELinux read/write performance
On Wed, 2007-09-12 at 17:51 +0900, Yuichi Nakamura wrote:
> Hi.
>
> Stephen Smalley pointed out possibility of race condition
> in off-list discussion.
> Stephen Smalley said:
> > One other observation about the patch: it presently leaves open a
> > (small) race window in which the file could get relabeled or policy gets
> > reloaded between the time of the normal permission check (from
> > selinux_inode_permission) and the time you copy the inode SID and policy
> > seqno to the file security struct. In which case you might end up never
> > revalidating access upon read/write even though conditions changed since
> > the open-time permission check. Not sure how to cleanly fix in a
> > lock-free manner, and adding locks here will only make matters worse.
>
> To fix that, permission has to be checked in selinux_dentry_open.
> Therefore, in open, number of permission checks increased.
> As shown in benchmark result below, it does not affect open/close
> performance so much.
>
> Following is benchmark result.
> * Benchmark
> lmbench simple read,write,open/close.
>
> * Before tuning
> 1) Result for x86(Pentium 4 2.6Ghz), kernel 2.6.22
> Base SELinux Overhead(%)
> Simple read 1.10 1.24 12.3
> Simple write 1.02 1.14 14.0
> open/close 5.97 7.45 24.9
> * Base: kernel compiled without SELinux support
>
> 2) Result for SH(SH4, SH7751R), kernel 2.6.22
> Base SELinux Overhead(%)
> Simple read 2.39 5.49 130.5
> Simple write 2.07 5.10 146.6
> open/close 32.6 62.8 93.0
>
> * After tuning
> 1) Result for x86(Pentium 4 2.6Ghz), kernel 2.6.22
> Base SELinux Overhead(%)
> Simple read 1.10 1.13 2.3(Before 12.3)
> Simple write 1.02 1.024 0.6(Before 14.0)
> open/close 5.97 7.48 25.3(Before 24.9)
> * Base: kernel compiled without SELinux support
>
> 2) Result for SH(SH4, SH7751R), kernel 2.6.22
> Base SELinux Overhead(%)
> Simple read 2.39 2.63 10.4(Before 130.5)
> Simple write 2.07 2.34 13.1(Before 146.6)
> open/close 32.6 58.7 80.2(before 93.0)
>
> Next is a patch.
Thanks, a few comments below.
>
> * Description of patch
> This patch improves performance of read/write in SELinux.
> It improves performance by skipping permission check in
> selinux_file_permission. Permission is only checked when
> sid change or policy load is detected after file open.
> To detect sid change, new LSM hook securiy_dentry_open is added.
I think I'd reword this a little, e.g.
It reduces the selinux overhead on read/write by only revalidating
permissions in selinux_file_permission if the task or inode labels have
changed or the policy has changed since the open-time check. A new LSM
hook, security_dentry_open, is added to capture the necessary state at
open time to allow this optimization.
>
> Signed-off-by: Yuichi Nakamura<ynakam@...achisoft.jp>
> ---
> fs/open.c | 5 ++++
> include/linux/security.h | 16 ++++++++++++++
> security/dummy.c | 6 +++++
> security/selinux/avc.c | 5 ++++
> security/selinux/hooks.c | 43 +++++++++++++++++++++++++++++++++++++-
> security/selinux/include/avc.h | 2 +
> security/selinux/include/objsec.h | 2 +
> 7 files changed, 78 insertions(+), 1 deletion(-)
<snip>
> diff -purN -X linux-2.6.22/Documentation/dontdiff linux-2.6.22.orig/security/selinux/hooks.c linux-2.6.22/security/selinux/hooks.c
> --- linux-2.6.22.orig/security/selinux/hooks.c 2007-07-09 08:32:17.000000000 +0900
> +++ linux-2.6.22/security/selinux/hooks.c 2007-09-12 08:42:49.000000000 +0900
> @@ -80,6 +82,7 @@
>
> #define XATTR_SELINUX_SUFFIX "selinux"
> #define XATTR_NAME_SELINUX XATTR_SECURITY_PREFIX XATTR_SELINUX_SUFFIX
> +#define ACC_MODE(x) ("\000\004\002\006"[(x)&O_ACCMODE])
Leftover from prior version of the patch, no longer needed.
<snip>
> @@ -2715,6 +2737,23 @@ static int selinux_file_receive(struct f
> return file_has_perm(current, file, file_to_av(file));
> }
>
> +static int selinux_dentry_open(struct file *file)
> +{
> + struct file_security_struct *fsec;
> + struct inode *inode;
> + struct inode_security_struct *isec;
> + inode = file->f_path.dentry->d_inode;
> + fsec = file->f_security;
> + isec = inode->i_security;
I'd add a comment here, e.g.
/*
* Save inode label and policy sequence number
* at open-time so that selinux_file_permission
* can determine whether revalidation is necessary.
* Task label is already saved in the file security
* struct as its SID.
*/
> + fsec->isid = isec->sid;
> + fsec->pseqno = avc_policy_seqno();
> +
> + /*Permission has to be rechecked here.
> + Policy load of inode sid can happen between
> + may_open and selinux_dentry_open.*/
Typo in the comment (s/of/or/), coding style isn't right for a
multi-line comment, and likely needs clarification, e.g.
/*
* Since the inode label or policy seqno may have changed
* between the selinux_inode_permission check and the saving
* of state above, recheck that access is still permitted.
* Otherwise, access might never be revalidated against the
* new inode label or new policy.
* This check is not redundant - do not remove.
*/
> + return inode_has_perm(current, inode, file_to_av(file), NULL);
> +}
> +
> /* task security operations */
>
> static int selinux_task_create(unsigned long clone_flags)
> diff -purN -X linux-2.6.22/Documentation/dontdiff linux-2.6.22.orig/fs/open.c linux-2.6.22/fs/open.c
> --- linux-2.6.22.orig/fs/open.c 2007-07-09 08:32:17.000000000 +0900
> +++ linux-2.6.22/fs/open.c 2007-09-12 08:31:24.000000000 +0900
> @@ -696,8 +696,13 @@ static struct file *__dentry_open(struct
> f->f_op = fops_get(inode->i_fop);
> file_move(f, &inode->i_sb->s_files);
>
> + error = security_dentry_open(f);
> + if (error)
> + goto cleanup_all;
> +
> if (!open && f->f_op)
> open = f->f_op->open;
> +
Extraneous whitespace leftover from prior version of the patch.
> if (open) {
> error = open(inode, f);
> if (error)
> diff -purN -X linux-2.6.22/Documentation/dontdiff linux-2.6.22.orig/include/linux/security.h linux-2.6.22/include/linux/security.h
> --- linux-2.6.22.orig/include/linux/security.h 2007-07-09 08:32:17.000000000 +0900
> +++ linux-2.6.22/include/linux/security.h 2007-09-12 08:30:16.000000000 +0900
> @@ -503,6 +503,11 @@ struct request_sock;
> * @file contains the file structure being received.
> * Return 0 if permission is granted.
> *
> + * Security hook for dentry
> + *
> + * @dentry_open
> + * Check permission or get additional information before opening dentry.
> + *
More precisely, "Save open-time permission checking state for later use
upon file_permission, and recheck access if anything has changed since
inode_permission."
--
Stephen Smalley
National Security Agency
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