ACK(ish): [Trusty][Xenial][PATCH 1/1] v3: CVE-2015-1350 fs: Avoid premature clearing of capabilities

[Stefan Bader]

On 13.12.2017 03:15, Khalid Elmously wrote:
> From: Jan Kara <jack at suse.cz>
> 
> 
> 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.
> 
> 
> (backported from commit 030b533c4fd4d2ec3402363323de4bb2983c9cee)
> [kmously: This is a re-implementation of the upstream commit that doesn't include all the refactoring that was done in upstream)
> 
> CVE-2015-1350
> 
> 
> References: CVE-2015-1350
> Reviewed-by: Christoph Hellwig <hch at lst.de>
> Signed-off-by: Jan Kara <jack at suse.cz>
> Signed-off-by: Khalid Elmously <khalid.elmously at canonical.com>
Acked-by: Stefan Bader <stefan.bader at canonical.com>

> 
> 
> 
> ---

Based on the suggestions from Thadeu this looks ok. Personally I am not 100%
sure whether the usage of d_obtain_alias() is valid or not.

-Stefan

>  fs/attr.c | 24 ++++++++++++++++++------
>  1 file changed, 18 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/attr.c b/fs/attr.c
> index df05bc167360..870d45103f29 100644
> --- a/fs/attr.c
> +++ b/fs/attr.c
> @@ -68,7 +68,7 @@ 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) && !chown_ok(inode, attr->ia_uid))
> @@ -95,6 +95,20 @@ 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;
> +
> +		dentry = d_obtain_alias(inode);
> +		if (!IS_ERR(dentry)) {
> +			error = security_inode_killpriv(dentry);
> +			dput(dentry);
> +			return error;
> +		}
> +	}
> +
>  	return 0;
>  }
>  EXPORT_SYMBOL(inode_change_ok);
> @@ -250,13 +264,11 @@ 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;
>  	}
>  
>  	/*
> 


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