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Message-Id: <1209655048.25678.478.camel@moss-spartans.epoch.ncsc.mil>
Date: Thu, 01 May 2008 11:17:28 -0400
From: Stephen Smalley <sds@...ho.nsa.gov>
To: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc: chrisw@...s-sol.org, viro@...IV.linux.org.uk,
akpm@...ux-foundation.org, linux-security-module@...r.kernel.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
takedakn@...data.co.jp, haradats@...data.co.jp
Subject: Re: [TOMOYO #8 (2.6.25-mm1) 1/7] Introduce new LSM hooks.
On Fri, 2008-05-02 at 00:07 +0900, Tetsuo Handa wrote:
> Hello.
>
> Chris Wright wrote:
> > * Toshiharu Harada (haradats@...data.co.jp) wrote:
> > > This patch allows LSM to check permission using "struct vfsmount"
> > > without passing "struct vfsmount" to VFS helper functions.
> >
> > This is simply duplicating many of the existing checks.
> > I don't see how this is an improvement.
> >
> > > --- mm.orig/fs/namei.c
> > > +++ mm/fs/namei.c
> > > @@ -1595,6 +1595,9 @@ int vfs_create(struct inode *dir, struct
> > > error = security_inode_create(dir, dentry, mode);
> > > if (error)
> > > return error;
> > > + error = security_path_create(dir, dentry, mode, nd);
> > > + if (error)
> > > + return error;
> >
> > Pure duplication (of course adding nameidata, although I think you just
> > want path).
>
> Stephen Smalley advised me to add parameter to existing hook rather than
> introducing a new hook if the location of existing hook is appropriate.
> OK. I'd like to add "struct nameidata" to security_inode_create()
> rather than introducing security_path_create() in the next patch.
I had thought you were going to add new hooks in the callers, not try to
use the nameidata here. And I wouldn't pass the nameidata there, just
what you actually need (e.g. the vfsmount).
> >
> > > DQUOT_INIT(dir);
> > > error = dir->i_op->create(dir, dentry, mode, nd);
> > > if (!error)
> > > @@ -1650,6 +1653,17 @@ int may_open(struct nameidata *nd, int a
> > ...
> > error = vfs_permission(nd, acc_mode);
> > if (error)
> > return error;
> > ...
> > > return -EPERM;
> > >
> > > /*
> > > + * security_inode_permission() called from vfs_permission()
> > > + * can't know that the file is going to be truncated when
> > > + * open(filename, O_WRONLY | O_TRUNC | O_APPEND) is used.
> > > + * So, this hook checks O_APPEND and O_TRUNC flags as well
> > > + * as MAY_READ and MAY_WRITE flags.
> > > + */
> > > + error = security_path_open(nd->path.dentry, nd->path.mnt, flag);
> > > + if (error)
> > > + return error;
> >
> > Also duplication. And why the unique flag handling, you don't seem to
> > ever check?
>
> The MAY_WRITE flag is not passed to security_inode_permission()
> if security_inode_permission() is called from __open_namei_create().
> Since TOMOYO Linux doesn't check MAY_READ/MAY_WRITE permissions for individual
> read()/write() requests, the permission checks at open() time (i.e. may_open())
> is the only chance to check MAY_WRITE flag. If I can't check MAY_WRITE flag
> here, TOMOYO Linux can't control open(O_WRONLY | O_CREATE | O_EXCL).
You can apply whatever checks you want from your hook in the create
path, right?
> > > @@ -2021,7 +2035,12 @@ fail:
> > > }
> > > EXPORT_SYMBOL_GPL(lookup_create);
> > >
> > > -int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
> > > +/*
> > > + * These pre_vfs_*() functions are separated from vfs_*() functions so that
> > > + * LSM's security_path_*() functions can do DAC checks before MAC checks
> > > + * without duplicating may_create()/may_delete() functions.
> > > + */
> > > +int pre_vfs_mknod(struct inode *dir, struct dentry *dentry, int mode)
> > > {
> > > int error = may_create(dir, dentry, NULL);
> > >
> > > @@ -2033,6 +2052,14 @@ int vfs_mknod(struct inode *dir, struct
> > >
> > > if (!dir->i_op || !dir->i_op->mknod)
> > > return -EPERM;
> > > + return 0;
> > > +}
> > > +
> > > +int vfs_mknod(struct inode *dir, struct dentry *dentry, int mode, dev_t dev)
> > > +{
> > > + int error = pre_vfs_mknod(dir, dentry, mode);
> > > + if (error)
> > > + return error;
> >
> > More duplication, you'll get a call chain like:
> >
> > sys_mknod
> > security_path_mknod
> > pre_vfs_mknod
> > vfs_mknod
> > pre_vfs_mknod
> > security_inode_mknod
> >
> This is an inevitable duplication since I want to do conventional checks
> (DAC checks and inode operation existence checks) before TOMOYO Linux's check.
>
> By the way, Stephen Smalley thinks it is better to copy codes which is needed by
> pre_vfs_*() (i.e. may_create()/may_delete()/check_sticky()) into
> security/tomoyo/ directory and leave vfs_*() untouched rather than
> extract pre_vfs_*() from vfs_*() and call pre_vfs_*() from vfs_*().
> Question to Al Viro: Do you prefer "copying may_create()/may_delete()/
> check_sticky() functions into security/tomoyo/ directory and leaving vfs_*()
> untouched" to "extracting pre_vfs_*() and making them accessible from
> security/tomoyo/ directory"? If you prefer copying, I'd like to copy them and
> remove pre_vfs_*() in the next patch.
I don't see how splitting out the parts that you are putting in the pre_
functions is especially useful. Making the may_create()/may_delete()
helpers non-static might make sense.
--
Stephen Smalley
National Security Agency
--
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