Re: [PATCH v4 2/20] lsm: Refactor return value of LSM hook inode_need_killpriv

Paul Moore
2024-07-19T02:08:02Z

On Jul 11, 2024 Xu Kuohai <xukuohai@huaweicloud.com> wrote:
>
> 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>
> ---
> 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(-)

In general I think a lot of these changes are a good improvement, thank
you very much for the time and effort you've spent on this. However,
I'm not in favor of passing the new hook parameter as a way of reducing
the number of states represented by the security_inode_killpriv() return
value. This particular hook may need to remain as one of the odd special
cases.

> 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

--
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