On Sat, Oct 10, 2015 at 7:59 AM, Dirk Steinmetz
<public@xxxxxxxxxxxxxxxxxx> wrote:
> Attempting to hardlink to an unsafe file (e.g. a setuid binary) from
> within an unprivileged user namespace fails, even if CAP_FOWNER is held
> within the namespace. This may cause various failures, such as a gentoo
> installation within a lxc container failing to build and install specific
> packages.
>
> This change permits hardlinking of files owned by mapped uids, if
> CAP_FOWNER is held for that namespace. Furthermore, it improves consistency
> by using the existing inode_owner_or_capable(), which is aware of
> namespaced capabilities as of 23adbe12ef7d3 ("fs,userns: Change
> inode_capable to capable_wrt_inode_uidgid").
>
> Signed-off-by: Dirk Steinmetz <public@xxxxxxxxxxxxxxxxxx>
Sorry for the delay: was travelling when I got put on CC. (FWIW, in
the future, please check the scripts/get_maintainer.pl script with
--git-blame to build CC lists, then I would have been CCed earlier.)
I think Eric's already taken this patch, but it looks correct to me:
Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>
I'll hop on the other thread to discuss the setgid issue.
-Kees
> ---
> fs/namei.c | 7 ++-----
> 1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/fs/namei.c b/fs/namei.c
> index 726d211..29fc6a6 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -955,26 +955,23 @@ static bool safe_hardlink_source(struct inode *inode)
> * - sysctl_protected_hardlinks enabled
> * - fsuid does not match inode
> * - hardlink source is unsafe (see safe_hardlink_source() above)
> - * - not CAP_FOWNER
> + * - not CAP_FOWNER in a namespace with the inode owner uid mapped
> *
> * Returns 0 if successful, -ve on error.
> */
> static int may_linkat(struct path *link)
> {
> - const struct cred *cred;
> struct inode *inode;
>
> if (!sysctl_protected_hardlinks)
> return 0;
>
> - cred = current_cred();
> inode = link->dentry->d_inode;
>
> /* Source inode owner (or CAP_FOWNER) can hardlink all they like,
> * otherwise, it must be a safe source.
> */
> - if (uid_eq(cred->fsuid, inode->i_uid) || safe_hardlink_source(inode) ||
> - capable(CAP_FOWNER))
> + if (inode_owner_or_capable(inode) || safe_hardlink_source(inode))
> return 0;
>
> audit_log_link_denied("linkat", link);
> --
> 2.1.4
>
> --
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--
Kees Cook
Chrome OS Security
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