Re: [RFC] namei: prevent sgid-hardlinks for unmapped gids

[Serge Hallyn <serge.hallyn@ubuntu.com>]

Quoting Dirk Steinmetz (public@rsjtdrjgfuzkfg.com):
> In order to hardlink to a sgid-executable, it is sufficient to be the
> file's owner. When hardlinking within an unprivileged user namespace, the
> users of that namespace could thus use hardlinks to pin setgid binaries
> owned by themselves (or any mapped uid, with CAP_FOWNER) and a gid outside
> of the namespace. This is a possible security risk.
> 
> This change prevents hardlinking of sgid-executables within user
> namespaces, if the file is not owned by a mapped gid.
> 
> Signed-off-by: Dirk Steinmetz <public@rsjtdrjgfuzkfg.com>

Hey,

Hoping this gets a close review by Kees, but this looks good to me, thanks!

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

> ---
> 
> MISSING: Documentation/sysctl/fs.txt not updated, as this patch is
> intended for discussion.
> 
> If there are no further misunderstandings on my side, this patch is what
> Serge and I agree on (modulo my not-that-much-linux-kernel-experience
> codestyle, feel free to suggest improvements!).
> 
> The new condition for sgid-executables is equivalent to
> > inode_owner_or_capable(inode) && kgid_has_mapping(ns, inode->i_gid)
> which, as recommended by Serge, does not change the behaviour for the init
> namespace. It fixes the problem of pinning parent namespace's gids.
> 
> However, I think the "same" security issue is also valid within any
> namespace, for regular users pinning other gids within the same namespace.
> I already presented an example for that in a previous mail:
> - A file has the setgid and user/group executable bits set, and is owned
>   by user:group.
> - The user 'user' is not in the group 'group', and does not have any
>   capabilities.
> - The user 'user' hardlinks the file. The permission check will succeed,
>   as the user is the owner of the file.
> - The file is replaced with a newer version (for example fixing a security
>   issue)
> - Now user can still use the hardlink-pinned version to execute the file
>   as 'user:group' (and for example exploit the security issue).
> 
> To prevent that, the condition would need to be changed to something like
> inode_group_or_capable, resembling inode_owner_or_capable, but checking
> that the caller is in the group the inode belongs to or has some
> capability (for consistency with former behaviour, CAP_FOWNER? for
> consistency with the documentation, CAP_FSETID?). However, this would
> change userland behaviour outside of userns. Thus my main question:
> Is the scenario above bad enough to change userland behaviour?
> 
> I'd apprechiate your comments.
> 
> - Dirk
> 
> 
> Diffstat:
>  fs/namei.c | 47 ++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 38 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 29fc6a6..9c6c2e2 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -913,18 +913,19 @@ static inline int may_follow_link(struct nameidata *nd)
>  }
>  
>  /**
> - * safe_hardlink_source - Check for safe hardlink conditions
> + * safe_hardlink_source_uid - Check for safe hardlink conditions not dependent
> + * on the inode's group. These conditions may be overridden by inode ownership
> + * or CAP_FOWNER with respect to the inode's uid
>   * @inode: the source inode to hardlink from
>   *
>   * Return false if at least one of the following conditions:
>   *    - inode is not a regular file
>   *    - inode is setuid
> - *    - inode is setgid and group-exec
>   *    - access failure for read and write
>   *
>   * Otherwise returns true.
>   */
> -static bool safe_hardlink_source(struct inode *inode)
> +static bool safe_hardlink_source_uid(struct inode *inode)
>  {
>  	umode_t mode = inode->i_mode;
>  
> @@ -936,10 +937,6 @@ static bool safe_hardlink_source(struct inode *inode)
>  	if (mode & S_ISUID)
>  		return false;
>  
> -	/* Executable setgid files should not get pinned to the filesystem. */
> -	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
> -		return false;
> -
>  	/* Hardlinking to unreadable or unwritable sources is dangerous. */
>  	if (inode_permission(inode, MAY_READ | MAY_WRITE))
>  		return false;
> @@ -948,30 +945,62 @@ static bool safe_hardlink_source(struct inode *inode)
>  }
>  
>  /**
> + * safe_hardlink_source_gid - Check for safe hardlink conditions dependent
> + * on the inode's group. These conditions may be overridden by inode ownership
> + * or CAP_FOWNER with respect to the inode's gid
> + * @inode: the source inode to hardlink from
> + *
> + * Return false if inode is setgid and group-exec
> + *
> + * Otherwise returns true.
> + */
> +static bool safe_hardlink_source_gid(struct inode *inode)
> +{
> +	umode_t mode = inode->i_mode;
> +
> +	/* Executable setgid files should not get pinned to the filesystem. */
> +	if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP))
> +		return false;
> +
> +	return true;
> +}
> +
> +/**
>   * may_linkat - Check permissions for creating a hardlink
>   * @link: the source to hardlink from
>   *
>   * Block hardlink when all of:
>   *  - sysctl_protected_hardlinks enabled
>   *  - fsuid does not match inode
> - *  - hardlink source is unsafe (see safe_hardlink_source() above)
> + *  - hardlink source is unsafe (see safe_hardlink_source_*() above)
>   *  - not CAP_FOWNER in a namespace with the inode owner uid mapped
> + *    (and inode gid mapped, if hardlink conditions depending on the inode's
> + *    group are not satisfied)
>   *
>   * Returns 0 if successful, -ve on error.
>   */
>  static int may_linkat(struct path *link)
>  {
>  	struct inode *inode;
> +	struct user_namespace *ns;
> +	bool owner;
> +	bool safe_uid;
> +	bool safe_gid;
>  
>  	if (!sysctl_protected_hardlinks)
>  		return 0;
>  
>  	inode = link->dentry->d_inode;
> +	ns = current_user_ns();
>  
>  	/* Source inode owner (or CAP_FOWNER) can hardlink all they like,
>  	 * otherwise, it must be a safe source.
>  	 */
> -	if (inode_owner_or_capable(inode) || safe_hardlink_source(inode))
> +	owner = inode_owner_or_capable(inode);
> +	safe_uid = safe_hardlink_source_uid(inode) || owner;
> +	safe_gid = safe_hardlink_source_gid(inode) ||
> +			(owner && kgid_has_mapping(ns, inode->i_gid));
> +	if (safe_uid && safe_gid)
>  		return 0;
>  
>  	audit_log_link_denied("linkat", link);
> -- 
> 2.1.4
>