[PATCH] security: Return xattr name from security_dentry_init_security()
Casey Schaufler
casey at schaufler-ca.com
Mon Oct 4 16:39:44 UTC 2021
On 10/4/2021 9:01 AM, Jeff Layton wrote:
> 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?
That's a bigger discussion than is probably appropriate on this thread.
In the module stacking case the caller needs to identify which security
module's context it wants. If the caller is capable of dealing with
multiple attributes (none currently are, but they all assume that you're
using SELinux and only support what SELinux needs) it will need to
do something different. We have chickens and eggs involved. The LSM
infrastructure doesn't need to handle it because none of its callers
are capable of dealing with it. None of the callers try, in part because
they have no way to get the information they would need, and in part
because they don't care about anything beyond SELinux. Ceph, for example,
is hard coded to expect "security.selinux".
On further reflection, Al's suggestion could be made to work if the
caller identified which attribute its looking for.
>>>> 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>
>>>
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