From f2ca379642d7a843be972ea4167abdd3c8c9e5d1 Mon Sep 17 00:00:00 2001 From: Dirk Steinmetz Date: Tue, 20 Oct 2015 16:09:19 +0200 Subject: [PATCH] namei: permit linking with CAP_FOWNER in userns 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 This is hitting us in Ubuntu during some dpkg upgrades in containers. When upgrading a file dpkg creates a hard link to the old file to back it up before overwriting it. When packages upgrade suid files owned by a non-root user the link isn't permitted, and the package upgrade fails. This patch fixes our problem. Tested-by: Seth Forshee Signed-off-by: Eric W. Biederman --- fs/namei.c | 7 ++----- 1 file changed, 2 insertions(+), 5 deletions(-) diff --git a/fs/namei.c b/fs/namei.c index 726d211db484..29fc6a657477 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);