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 <public@rsjtdrjgfuzkfg.com>
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 <seth.forshee@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
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 @@
* - 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);