Re: [PATCH bpf-next v4 02/20] lsm: Refactor return value of LSM hook inode_need_killpriv

Serge Hallyn
2024-07-11T14:16:22Z

Jul 11, 2024 06:14:09 Xu Kuohai <xukuohai@huaweicloud.com>:

> From: Xu Kuohai <xukuohai@huawei.com>
>
> To be consistent with most LSM hooks, convert the return value of
> hook inode_need_killpriv to 0 or a negative error code.
>
> Before:
> - Both hook inode_need_killpriv and func security_inode_need_killpriv
>   return > 0 if security_inode_killpriv is required, 0 if not, and < 0
>   to abort the operation.
>
> After:
> - Both hook inode_need_killpriv and func security_inode_need_killpriv
>   return 0 on success and a negative error code on failure.
>   On success, hook inode_need_killpriv sets output param @need to true
>   if security_inode_killpriv is required, and false if not. When @need
>   is true, func security_inode_need_killpriv sets ATTR_KILL_PRIV flag
>   in @attr; when false, it clears the flag.
>   On failure, @need and @attr remains unchanged.
>
> Signed-off-by: Xu Kuohai <xukuohai@huawei.com>

It looks ok - though unnecessary (I'm assuming a later patch works better with this) - , but I'd be more comfortable if it was documented that any callers of the need_killpriv hook must set need to false before calling. Or if the hooks set need to false at start.

> ---
> fs/attr.c                     |  5 ++---
> fs/inode.c                    |  4 +---
> include/linux/lsm_hook_defs.h |  2 +-
> include/linux/security.h      | 20 ++++++++++++++++----
> security/commoncap.c          | 12 ++++++++----
> security/security.c           | 29 ++++++++++++++++++++++++-----
> 6 files changed, 52 insertions(+), 20 deletions(-)
>
> diff --git a/fs/attr.c b/fs/attr.c
> index 960a310581eb..aaadc721c982 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -427,11 +427,10 @@ int notify_change(struct mnt_idmap *idmap, struct dentry *dentry,
>         attr->ia_mtime = timestamp_truncate(attr->ia_mtime, inode);
>
>     if (ia_valid & ATTR_KILL_PRIV) {
> -       error = security_inode_need_killpriv(dentry);
> +       error = security_inode_need_killpriv(dentry, &ia_valid);
>         if (error < 0)
>             return error;
> -       if (error == 0)
> -           ia_valid = attr->ia_valid &= ~ATTR_KILL_PRIV;
> +       attr->ia_valid = ia_valid;
>     }
>
>     /*
> diff --git a/fs/inode.c b/fs/inode.c
> index 3a41f83a4ba5..cd335dc3a3bc 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -2012,11 +2012,9 @@ int dentry_needs_remove_privs(struct mnt_idmap *idmap,
>         return 0;
>
>     mask = setattr_should_drop_suidgid(idmap, inode);
> -   ret = security_inode_need_killpriv(dentry);
> +   ret = security_inode_need_killpriv(dentry, &mask);
>     if (ret < 0)
>         return ret;
> -   if (ret)
> -       mask |= ATTR_KILL_PRIV;
>     return mask;
> }
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index e6e6f8473955..964849de424b 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -165,7 +165,7 @@ LSM_HOOK(int, 0, inode_remove_acl, struct mnt_idmap *idmap,
>      struct dentry *dentry, const char *acl_name)
> LSM_HOOK(void, LSM_RET_VOID, inode_post_remove_acl, struct mnt_idmap *idmap,
>      struct dentry *dentry, const char *acl_name)
> -LSM_HOOK(int, 0, inode_need_killpriv, struct dentry *dentry)
> +LSM_HOOK(int, 0, inode_need_killpriv, struct dentry *dentry, bool *need)
> LSM_HOOK(int, 0, inode_killpriv, struct mnt_idmap *idmap,
>      struct dentry *dentry)
> LSM_HOOK(int, -EOPNOTSUPP, inode_getsecurity, struct mnt_idmap *idmap,
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 454f96307cb9..1614ef5b2dd2 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -161,7 +161,7 @@ int cap_inode_setxattr(struct dentry *dentry, const char *name,
>                const void *value, size_t size, int flags);
> int cap_inode_removexattr(struct mnt_idmap *idmap,
>               struct dentry *dentry, const char *name);
> -int cap_inode_need_killpriv(struct dentry *dentry);
> +int cap_inode_need_killpriv(struct dentry *dentry, bool *need);
> int cap_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry);
> int cap_inode_getsecurity(struct mnt_idmap *idmap,
>               struct inode *inode, const char *name, void **buffer,
> @@ -389,7 +389,7 @@ int security_inode_listxattr(struct dentry *dentry);
> int security_inode_removexattr(struct mnt_idmap *idmap,
>                    struct dentry *dentry, const char *name);
> void security_inode_post_removexattr(struct dentry *dentry, const char *name);
> -int security_inode_need_killpriv(struct dentry *dentry);
> +int security_inode_need_killpriv(struct dentry *dentry, int *attr);
> int security_inode_killpriv(struct mnt_idmap *idmap, struct dentry *dentry);
> int security_inode_getsecurity(struct mnt_idmap *idmap,
>                    struct inode *inode, const char *name,
> @@ -971,9 +971,21 @@ static inline void security_inode_post_removexattr(struct dentry *dentry,
>                            const char *name)
> { }
>
> -static inline int security_inode_need_killpriv(struct dentry *dentry)
> +static inline int security_inode_need_killpriv(struct dentry *dentry, int *attr)
> {
> -   return cap_inode_need_killpriv(dentry);
> +   int rc;
> +   bool need = false;
> +
> +   rc = cap_inode_need_killpriv(dentry, &need);
> +   if (rc < 0)
> +       return rc;
> +
> +   if (need)
> +       *attr |= ATTR_KILL_PRIV;
> +   else
> +       *attr &= ~ATTR_KILL_PRIV;
> +
> +   return 0;
> }
>
> static inline int security_inode_killpriv(struct mnt_idmap *idmap,
> diff --git a/security/commoncap.c b/security/commoncap.c
> index cefad323a0b1..17d6188d22cf 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -286,21 +286,25 @@ int cap_capset(struct cred *new,
> /**
>   * cap_inode_need_killpriv - Determine if inode change affects privileges
>   * @dentry: The inode/dentry in being changed with change marked ATTR_KILL_PRIV
> + * @need: If inode_killpriv() is needed
>   *
>   * Determine if an inode having a change applied that's marked ATTR_KILL_PRIV
>   * affects the security markings on that inode, and if it is, should
>   * inode_killpriv() be invoked or the change rejected.
>   *
> - * Return: 1 if security.capability has a value, meaning inode_killpriv()
> - * is required, 0 otherwise, meaning inode_killpriv() is not required.
> + * Return: Always returns 0. If security.capability has a value, meaning
> + * inode_killpriv() is required, @need is set to true.
>   */
> -int cap_inode_need_killpriv(struct dentry *dentry)
> +int cap_inode_need_killpriv(struct dentry *dentry, bool *need)
> {
>     struct inode *inode = d_backing_inode(dentry);
>     int error;
>
>     error = __vfs_getxattr(dentry, inode, XATTR_NAME_CAPS, NULL, 0);
> -   return error > 0;
> +   if (error > 0)
> +       *need = true;
> +
> +   return 0;
> }
>
> /**
> diff --git a/security/security.c b/security/security.c
> index 3475f0cab3da..a4abcd86eb36 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -2490,17 +2490,36 @@ void security_inode_post_removexattr(struct dentry *dentry, const char *name)
> /**
>   * security_inode_need_killpriv() - Check if security_inode_killpriv() required
>   * @dentry: associated dentry
> + * @attr: attribute flags
>   *
>   * Called when an inode has been changed to determine if
>   * security_inode_killpriv() should be called.
>   *
> - * Return: Return <0 on error to abort the inode change operation, return 0 if
> - *         security_inode_killpriv() does not need to be called, return >0 if
> - *         security_inode_killpriv() does need to be called.
> + * Return: Return 0 on success, negative error code on failure.
> + *         On success, set ATTR_KILL_PRIV flag in @attr when @need is true,
> + *         clears it when false.
>   */
> -int security_inode_need_killpriv(struct dentry *dentry)
> +int security_inode_need_killpriv(struct dentry *dentry, int *attr)
> {
> -   return call_int_hook(inode_need_killpriv, dentry);
> +   int rc;
> +   bool need = false;
> +   struct security_hook_list *hp;
> +
> +   hlist_for_each_entry(hp, &security_hook_heads.inode_need_killpriv,
> +                list) {
> +       rc = hp->hook.inode_need_killpriv(dentry, &need);
> +       if (rc < 0)
> +           return rc;
> +       if (need)
> +           break;
> +   }
> +
> +   if (need)
> +       *attr |= ATTR_KILL_PRIV;
> +   else
> +       *attr &= ~ATTR_KILL_PRIV;
> +
> +   return 0;
> }
>
> /**
> --
> 2.30.2