[PATCH] security: Return xattr name from security_dentry_init_security()
Jeff Layton
jlayton at kernel.org
Mon Oct 4 16:01:24 UTC 2021
On Mon, 2021-10-04 at 08:54 -0700, Casey Schaufler wrote:
> On 10/4/2021 8:20 AM, Jeff Layton wrote:
> > On Thu, 2021-09-30 at 14:59 -0400, Vivek Goyal wrote:
> > > Right now security_dentry_init_security() only supports single security
> > > label and is used by SELinux only. There are two users of of this hook,
> > > namely ceph and nfs.
> > >
> > > NFS does not care about xattr name. Ceph hardcodes the xattr name to
> > > security.selinux (XATTR_NAME_SELINUX).
> > >
> > > I am making changes to fuse/virtiofs to send security label to virtiofsd
> > > and I need to send xattr name as well. I also hardcoded the name of
> > > xattr to security.selinux.
> > >
> > > Stephen Smalley suggested that it probably is a good idea to modify
> > > security_dentry_init_security() to also return name of xattr so that
> > > we can avoid this hardcoding in the callers.
> > >
> > > This patch adds a new parameter "const char **xattr_name" to
> > > security_dentry_init_security() and LSM puts the name of xattr
> > > too if caller asked for it (xattr_name != NULL).
> > >
> > > Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> > > ---
> > >
> > > I have compile tested this patch. Don't know how to setup ceph and
> > > test it. Its a very simple change. Hopefully ceph developers can
> > > have a quick look at it.
> > >
> > > A similar attempt was made three years back.
> > >
> > > https://lore.kernel.org/linux-security-module/20180626080429.27304-1-zyan@redhat.com/T/
> > > ---
> > > fs/ceph/xattr.c | 3 +--
> > > fs/nfs/nfs4proc.c | 3 ++-
> > > include/linux/lsm_hook_defs.h | 3 ++-
> > > include/linux/lsm_hooks.h | 1 +
> > > include/linux/security.h | 6 ++++--
> > > security/security.c | 7 ++++---
> > > security/selinux/hooks.c | 6 +++++-
> > > 7 files changed, 19 insertions(+), 10 deletions(-)
> > >
> > > Index: redhat-linux/security/selinux/hooks.c
> > > ===================================================================
> > > --- redhat-linux.orig/security/selinux/hooks.c 2021-09-28 11:36:03.559785943 -0400
> > > +++ redhat-linux/security/selinux/hooks.c 2021-09-30 14:01:05.869195347 -0400
> > > @@ -2948,7 +2948,8 @@ static void selinux_inode_free_security(
> > > }
> > >
> > I agree with Al that it would be cleaner to just return the string, but
> > the call_*_hook stuff makes that a bit more tricky. I suppose this is a
> > reasonable compromise.
>
> call_int_hook() and call_void_hook() were introduced to reduce the monotonous
> repetition in the source. They are cosmetic and add no real value. They shouldn't
> be a consideration in the discussion.
>
> There is a problem with Al's suggestion. The interface as used today has two real
> problems. It returns an attribute value without identifying the attribute. Al's
> interface would address this issue. The other problem is that the interface can't
> provide multiple attribute+value pairs. The interface is going to need changed to
> support that for full module stacking. I don't see a rational way to extend the
> interface if it returns a string when there are multiple attributes to choose from.
>
Is that also a problem for the ctx parameter? In the case of full module
stacking do you get back multiple contexts as well?
> > > static int selinux_dentry_init_security(struct dentry *dentry, int mode,
> > > - const struct qstr *name, void **ctx,
> > > + const struct qstr *name,
> > > + const char **xattr_name, void **ctx,
> > > u32 *ctxlen)
> > > {
> > > u32 newsid;
> > > @@ -2961,6 +2962,9 @@ static int selinux_dentry_init_security(
> > > if (rc)
> > > return rc;
> > >
> > > + if (xattr_name)
> > > + *xattr_name = XATTR_NAME_SELINUX;
> > > +
> > > return security_sid_to_context(&selinux_state, newsid, (char **)ctx,
> > > ctxlen);
> > > }
> > > Index: redhat-linux/security/security.c
> > > ===================================================================
> > > --- redhat-linux.orig/security/security.c 2021-08-16 10:39:28.518988836 -0400
> > > +++ redhat-linux/security/security.c 2021-09-30 13:54:36.367195347 -0400
> > > @@ -1052,11 +1052,12 @@ void security_inode_free(struct inode *i
> > > }
> > >
> > > int security_dentry_init_security(struct dentry *dentry, int mode,
> > > - const struct qstr *name, void **ctx,
> > > - u32 *ctxlen)
> > > + const struct qstr *name,
> > > + const char **xattr_name, void **ctx,
> > > + u32 *ctxlen)
> > > {
> > > return call_int_hook(dentry_init_security, -EOPNOTSUPP, dentry, mode,
> > > - name, ctx, ctxlen);
> > > + name, xattr_name, ctx, ctxlen);
> > > }
> > > EXPORT_SYMBOL(security_dentry_init_security);
> > >
> > > Index: redhat-linux/include/linux/lsm_hooks.h
> > > ===================================================================
> > > --- redhat-linux.orig/include/linux/lsm_hooks.h 2021-06-02 10:20:27.717485143 -0400
> > > +++ redhat-linux/include/linux/lsm_hooks.h 2021-09-30 13:56:48.440195347 -0400
> > > @@ -196,6 +196,7 @@
> > > * @dentry dentry to use in calculating the context.
> > > * @mode mode used to determine resource type.
> > > * @name name of the last path component used to create file
> > > + * @xattr_name pointer to place the pointer to security xattr name
> > It might be a good idea to also document the lifetime for xattr_name
> > here. In particular you're returning a pointer to a static string, and
> > it would be good to note that the caller needn't free it or anything.
> >
> > > * @ctx pointer to place the pointer to the resulting context in.
> > > * @ctxlen point to place the length of the resulting context.
> > > * @dentry_create_files_as:
> > > Index: redhat-linux/include/linux/security.h
> > > ===================================================================
> > > --- redhat-linux.orig/include/linux/security.h 2021-08-16 10:39:28.484988836 -0400
> > > +++ redhat-linux/include/linux/security.h 2021-09-30 13:59:00.288195347 -0400
> > > @@ -317,8 +317,9 @@ int security_add_mnt_opt(const char *opt
> > > int len, void **mnt_opts);
> > > int security_move_mount(const struct path *from_path, const struct path *to_path);
> > > int security_dentry_init_security(struct dentry *dentry, int mode,
> > > - const struct qstr *name, void **ctx,
> > > - u32 *ctxlen);
> > > + const struct qstr *name,
> > > + const char **xattr_name, void **ctx,
> > > + u32 *ctxlen);
> > > int security_dentry_create_files_as(struct dentry *dentry, int mode,
> > > struct qstr *name,
> > > const struct cred *old,
> > > @@ -739,6 +740,7 @@ static inline void security_inode_free(s
> > > static inline int security_dentry_init_security(struct dentry *dentry,
> > > int mode,
> > > const struct qstr *name,
> > > + const char **xattr_name,
> > > void **ctx,
> > > u32 *ctxlen)
> > > {
> > > Index: redhat-linux/include/linux/lsm_hook_defs.h
> > > ===================================================================
> > > --- redhat-linux.orig/include/linux/lsm_hook_defs.h 2021-07-07 11:54:59.673549151 -0400
> > > +++ redhat-linux/include/linux/lsm_hook_defs.h 2021-09-30 14:02:13.114195347 -0400
> > > @@ -83,7 +83,8 @@ LSM_HOOK(int, 0, sb_add_mnt_opt, const c
> > > LSM_HOOK(int, 0, move_mount, const struct path *from_path,
> > > const struct path *to_path)
> > > LSM_HOOK(int, 0, dentry_init_security, struct dentry *dentry,
> > > - int mode, const struct qstr *name, void **ctx, u32 *ctxlen)
> > > + int mode, const struct qstr *name, const char **xattr_name,
> > > + void **ctx, u32 *ctxlen)
> > > LSM_HOOK(int, 0, dentry_create_files_as, struct dentry *dentry, int mode,
> > > struct qstr *name, const struct cred *old, struct cred *new)
> > >
> > > Index: redhat-linux/fs/nfs/nfs4proc.c
> > > ===================================================================
> > > --- redhat-linux.orig/fs/nfs/nfs4proc.c 2021-07-14 14:47:42.732842926 -0400
> > > +++ redhat-linux/fs/nfs/nfs4proc.c 2021-09-30 14:06:02.249195347 -0400
> > > @@ -127,7 +127,8 @@ nfs4_label_init_security(struct inode *d
> > > return NULL;
> > >
> > > err = security_dentry_init_security(dentry, sattr->ia_mode,
> > > - &dentry->d_name, (void **)&label->label, &label->len);
> > > + &dentry->d_name, NULL,
> > > + (void **)&label->label, &label->len);
> > > if (err == 0)
> > > return label;
> > >
> > > Index: redhat-linux/fs/ceph/xattr.c
> > > ===================================================================
> > > --- redhat-linux.orig/fs/ceph/xattr.c 2021-09-09 13:05:21.800611264 -0400
> > > +++ redhat-linux/fs/ceph/xattr.c 2021-09-30 14:14:59.892195347 -0400
> > > @@ -1311,7 +1311,7 @@ int ceph_security_init_secctx(struct den
> > > int err;
> > >
> > > err = security_dentry_init_security(dentry, mode, &dentry->d_name,
> > > - &as_ctx->sec_ctx,
> > > + &name, &as_ctx->sec_ctx,
> > > &as_ctx->sec_ctxlen);
> > > if (err < 0) {
> > > WARN_ON_ONCE(err != -EOPNOTSUPP);
> > > @@ -1335,7 +1335,6 @@ int ceph_security_init_secctx(struct den
> > > * It only supports single security module and only selinux has
> > > * dentry_init_security hook.
> > > */
> > > - name = XATTR_NAME_SELINUX;
> > > name_len = strlen(name);
> > > err = ceph_pagelist_reserve(pagelist,
> > > 4 * 2 + name_len + as_ctx->sec_ctxlen);
> > >
> > Looks reasonable overall.
> >
> > Reviewed-by: Jeff Layton <jlayton at kernel.org>
> >
>
--
Jeff Layton <jlayton at kernel.org>
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