[PATCH] security: Fix hook iteration and default value for inode_copy_up_xattr

[PATCH] security: Fix hook iteration and default value for inode_copy_up_xattr

[KP Singh]

From: KP Singh <kpsingh@google.com>

inode_copy_up_xattr returns 0 to indicate the acceptance of the xattr
and 1 to reject it. If the LSM does not know about the xattr, it's
expected to return -EOPNOTSUPP, which is the correct default value for
this hook. BPF LSM, currently, uses 0 as the default value and thereby
falsely allows all overlay fs xattributes to be copied up.

The iteration logic is also updated from the "bail-on-fail"
call_int_hook to continue on the non-decisive -EOPNOTSUPP and bail out
on other values.

Fixes: 98e828a0650f ("security: Refactor declaration of LSM hooks")
Signed-off-by: KP Singh <kpsingh@google.com>
---
 include/linux/lsm_hook_defs.h |  2 +-
 security/security.c           | 17 ++++++++++++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
index 6791813cd439..f4b2e54162ae 100644
--- a/include/linux/lsm_hook_defs.h
+++ b/include/linux/lsm_hook_defs.h
@@ -150,7 +150,7 @@ LSM_HOOK(int, 0, inode_listsecurity, struct inode *inode, char *buffer,
 	 size_t buffer_size)
 LSM_HOOK(void, LSM_RET_VOID, inode_getsecid, struct inode *inode, u32 *secid)
 LSM_HOOK(int, 0, inode_copy_up, struct dentry *src, struct cred **new)
-LSM_HOOK(int, 0, inode_copy_up_xattr, const char *name)
+LSM_HOOK(int, -EOPNOTSUPP, inode_copy_up_xattr, const char *name)
 LSM_HOOK(int, 0, kernfs_init_security, struct kernfs_node *kn_dir,
 	 struct kernfs_node *kn)
 LSM_HOOK(int, 0, file_permission, struct file *file, int mask)
diff --git a/security/security.c b/security/security.c
index 0ce3e73edd42..70a7ad357bc6 100644
--- a/security/security.c
+++ b/security/security.c
@@ -1414,7 +1414,22 @@ EXPORT_SYMBOL(security_inode_copy_up);
 
 int security_inode_copy_up_xattr(const char *name)
 {
-	return call_int_hook(inode_copy_up_xattr, -EOPNOTSUPP, name);
+	struct security_hook_list *hp;
+	int rc;
+
+	/*
+	 * The implementation can return 0 (accept the xattr), 1 (discard the
+	 * xattr), -EOPNOTSUPP if it does not know anything about the xattr or
+	 * any other error code incase of an error.
+	 */
+	hlist_for_each_entry(hp,
+		&security_hook_heads.inode_copy_up_xattr, list) {
+		rc = hp->hook.inode_copy_up_xattr(name);
+		if (rc != LSM_RET_DEFAULT(inode_copy_up_xattr))
+			return rc;
+	}
+
+	return LSM_RET_DEFAULT(inode_copy_up_xattr);
 }
 EXPORT_SYMBOL(security_inode_copy_up_xattr);
 
-- 
2.27.0.111.gc72c7da667-goog

Re: [PATCH] security: Fix hook iteration and default value for inode_copy_up_xattr

[James Morris]

On Mon, 22 Jun 2020, KP Singh wrote:

> From: KP Singh <kpsingh@google.com>
> 
> inode_copy_up_xattr returns 0 to indicate the acceptance of the xattr
> and 1 to reject it. If the LSM does not know about the xattr, it's
> expected to return -EOPNOTSUPP, which is the correct default value for
> this hook. BPF LSM, currently, uses 0 as the default value and thereby
> falsely allows all overlay fs xattributes to be copied up.
> 
> The iteration logic is also updated from the "bail-on-fail"
> call_int_hook to continue on the non-decisive -EOPNOTSUPP and bail out
> on other values.
> 
> Fixes: 98e828a0650f ("security: Refactor declaration of LSM hooks")
> Signed-off-by: KP Singh <kpsingh@google.com>

Applied to
git://git.kernel.org/pub/scm/linux/kernel/git/jmorris/linux-security.git fixes-v5.8

> ---
>  include/linux/lsm_hook_defs.h |  2 +-
>  security/security.c           | 17 ++++++++++++++++-
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 6791813cd439..f4b2e54162ae 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -150,7 +150,7 @@ LSM_HOOK(int, 0, inode_listsecurity, struct inode *inode, char *buffer,
>  	 size_t buffer_size)
>  LSM_HOOK(void, LSM_RET_VOID, inode_getsecid, struct inode *inode, u32 *secid)
>  LSM_HOOK(int, 0, inode_copy_up, struct dentry *src, struct cred **new)
> -LSM_HOOK(int, 0, inode_copy_up_xattr, const char *name)
> +LSM_HOOK(int, -EOPNOTSUPP, inode_copy_up_xattr, const char *name)
>  LSM_HOOK(int, 0, kernfs_init_security, struct kernfs_node *kn_dir,
>  	 struct kernfs_node *kn)
>  LSM_HOOK(int, 0, file_permission, struct file *file, int mask)
> diff --git a/security/security.c b/security/security.c
> index 0ce3e73edd42..70a7ad357bc6 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -1414,7 +1414,22 @@ EXPORT_SYMBOL(security_inode_copy_up);
>  
>  int security_inode_copy_up_xattr(const char *name)
>  {
> -	return call_int_hook(inode_copy_up_xattr, -EOPNOTSUPP, name);
> +	struct security_hook_list *hp;
> +	int rc;
> +
> +	/*
> +	 * The implementation can return 0 (accept the xattr), 1 (discard the
> +	 * xattr), -EOPNOTSUPP if it does not know anything about the xattr or
> +	 * any other error code incase of an error.
> +	 */
> +	hlist_for_each_entry(hp,
> +		&security_hook_heads.inode_copy_up_xattr, list) {
> +		rc = hp->hook.inode_copy_up_xattr(name);
> +		if (rc != LSM_RET_DEFAULT(inode_copy_up_xattr))
> +			return rc;
> +	}
> +
> +	return LSM_RET_DEFAULT(inode_copy_up_xattr);
>  }
>  EXPORT_SYMBOL(security_inode_copy_up_xattr);
>  
> 

-- 
James Morris
<jmorris@namei.org>