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    SubjectRe: [TOMOYO #8 (2.6.25-mm1) 1/7] Introduce new LSM hooks.
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    On Fri, 2008-05-02 at 00:07 +0900, Tetsuo Handa wrote:
    > Hello.
    >
    > Chris Wright wrote:
    > > * Toshiharu Harada (haradats@nttdata.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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