[T,CVE-2015-1350] fs: Avoid premature clearing of capabilities

CVE-2015-1350

Buglink: https://launchpad.net/bugs/1415636

Currently, notify_change() clears capabilities or IMA attributes by
calling security_inode_killpriv() before calling into ->setattr. Thus it
happens before any other permission checks in inode_change_ok() and user
is thus allowed to trigger clearing of capabilities or IMA attributes
for any file he can look up e.g. by calling chown for that file. This is
unexpected and can lead to user DoSing a system.

Fix the problem by calling security_inode_killpriv() at the end of
inode_change_ok() instead of from notify_change(). At that moment we are
sure user has permissions to do the requested change.

Signed-off-by: José Pekkarinen <jose.pekkarinen@canonical.com>
---
 fs/attr.c | 23 +++++++++++++++++------
 1 file changed, 17 insertions(+), 6 deletions(-)

On 08.08.2018 15:46, José Pekkarinen wrote:
> CVE-2015-1350
> 
> Buglink: https://launchpad.net/bugs/1415636
> 
> Currently, notify_change() clears capabilities or IMA attributes by
> calling security_inode_killpriv() before calling into ->setattr. Thus it
> happens before any other permission checks in inode_change_ok() and user
> is thus allowed to trigger clearing of capabilities or IMA attributes
> for any file he can look up e.g. by calling chown for that file. This is
> unexpected and can lead to user DoSing a system.
> 
> Fix the problem by calling security_inode_killpriv() at the end of
> inode_change_ok() instead of from notify_change(). At that moment we are
> sure user has permissions to do the requested change.
> 
> Signed-off-by: José Pekkarinen <jose.pekkarinen@canonical.com>

Same reasons as for the Xenial submission.

-Stefan
> ---
>  fs/attr.c | 23 +++++++++++++++++------
>  1 file changed, 17 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index 6530ced..f58bbc8 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -44,7 +44,7 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
>  
>  	/* If force is set do it anyway. */
>  	if (ia_valid & ATTR_FORCE)
> -		return 0;
> +		goto kill_priv;
>  
>  	/* Make sure a caller can chown. */
>  	if ((ia_valid & ATTR_UID) &&
> @@ -77,6 +77,19 @@ int inode_change_ok(const struct inode *inode, struct iattr *attr)
>  			return -EPERM;
>  	}
>  
> +kill_priv:
> +	/* User has permission for the change */
> +	if (ia_valid & ATTR_KILL_PRIV) {
> +		int error;
> +		struct dentry * dentry = d_find_alias(inode);
> +
> +		if (dentry) {
> +			error = security_inode_killpriv(dentry);
> +			if (error)
> +				return error;
> +		}
> +	}
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL(inode_change_ok);
> @@ -217,13 +230,11 @@ int notify_change(struct dentry * dentry, struct iattr * attr, struct inode **de
>  	if (!(ia_valid & ATTR_MTIME_SET))
>  		attr->ia_mtime = now;
>  	if (ia_valid & ATTR_KILL_PRIV) {
> -		attr->ia_valid &= ~ATTR_KILL_PRIV;
> -		ia_valid &= ~ATTR_KILL_PRIV;
>  		error = security_inode_need_killpriv(dentry);
> -		if (error > 0)
> -			error = security_inode_killpriv(dentry);
> -		if (error)
> +		if (error < 0)
>  			return error;
> +		if (error == 0)
> +			ia_valid = attr->ia_valid &= ~ATTR_KILL_PRIV;
>  	}
>  
>  	/*
>